ceph: remove the extra slashes in the server path

ceph: remove the extra slashes in the server path

It's possible to pass the mount helper a server path that has more
than one contiguous slash character. For example:

    mount -t ceph 192.168.195.165:40176:/// /mnt/cephfs/

In the MDS server side the extra slashes of the server path will be
treated as snap dir, and then we can get the following debug logs:

<7>[  ...] ceph:  mount opening path //
<7>[  ...] ceph:  open_root_inode opening '//'
<7>[  ...] ceph:  fill_trace 0000000059b8a3bc is_dentry 0 is_target 1
<7>[  ...] ceph:  alloc_inode 00000000dc4ca00b
<7>[  ...] ceph:  get_inode created new inode 00000000dc4ca00b 1.ffffffffffffffff ino 1
<7>[  ...] ceph:  get_inode on 1=1.ffffffffffffffff got 00000000dc4ca00b

And then when creating any new file or directory under the mount
point, we can get the following crash core dump:

<4>[  ...] ------------[ cut here ]------------
<2>[  ...] kernel BUG at fs/ceph/inode.c:1347!
<4>[  ...] invalid opcode: 0000 [#1] SMP PTI
<4>[  ...] CPU: 0 PID: 7 Comm: kworker/0:1 Tainted: G            E     5.4.0-rc5+ #1
<4>[  ...] Hardware name: VMware, Inc. VMware Virtual Platform/440BX Desktop Reference Platform, BIOS 6.00 05/19/2017
<4>[  ...] Workqueue: ceph-msgr ceph_con_workfn [libceph]
<4>[  ...] RIP: 0010:ceph_fill_trace+0x992/0xb30 [ceph]
<4>[  ...] Code: ff 0f 0b 0f 0b 0f 0b 4c 89 fa 48 c7 c6 4d [...]
<4>[  ...] RSP: 0018:ffffa23d40067c70 EFLAGS: 00010297
<4>[  ...] RAX: fffffffffffffffe RBX: ffff8a229eb566c0 RCX: 0000000000000006
<4>[  ...] RDX: 0000000000000000 RSI: 0000000000000092 RDI: ffff8a23aec17900
<4>[  ...] RBP: ffff8a226bd91eb0 R08: 0000000000000001 R09: 0000000000000885
<4>[  ...] R10: 000000000002dfd8 R11: ffff8a226bd95b30 R12: ffff8a239347e000
<4>[  ...] R13: 0000000000000000 R14: ffff8a22fabeb000 R15: ffff8a2338b0c900
<4>[  ...] FS:  0000000000000000(0000) GS:ffff8a23aec00000(0000) knlGS:0000000000000000
<4>[  ...] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
<4>[  ...] CR2: 000055b479d92068 CR3: 00000003764f6004 CR4: 00000000003606f0
<4>[  ...] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
<4>[  ...] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
<4>[  ...] Call Trace:
<4>[  ...]  dispatch+0x2ac/0x12b0 [ceph]
<4>[  ...]  ceph_con_workfn+0xd40/0x27c0 [libceph]
<4>[  ...]  process_one_work+0x1b0/0x350
<4>[  ...]  worker_thread+0x50/0x3b0
<4>[  ...]  kthread+0xfb/0x130
<4>[  ...]  ? process_one_work+0x350/0x350
<4>[  ...]  ? kthread_park+0x90/0x90
<4>[  ...]  ret_from_fork+0x35/0x40
<4>[  ...] Modules linked in: ceph(E) libceph fscache [...]
<4>[  ...] ---[ end trace ba883d8ccf9afcb0 ]---
<4>[  ...] RIP: 0010:ceph_fill_trace+0x992/0xb30 [ceph]
<4>[  ...] Code: ff 0f 0b 0f 0b 0f 0b 4c 89 fa 48 c7 c6 [...]
<4>[  ...] RSP: 0018:ffffa23d40067c70 EFLAGS: 00010297
<4>[  ...] RAX: fffffffffffffffe RBX: ffff8a229eb566c0 RCX: 0000000000000006
<4>[  ...] RDX: 0000000000000000 RSI: 0000000000000092 RDI: ffff8a23aec17900
<4>[  ...] RBP: ffff8a226bd91eb0 R08: 0000000000000001 R09: 0000000000000885
<4>[  ...] R10: 000000000002dfd8 R11: ffff8a226bd95b30 R12: ffff8a239347e000
<4>[  ...] R13: 0000000000000000 R14: ffff8a22fabeb000 R15: ffff8a2338b0c900
<4>[  ...] FS:  0000000000000000(0000) GS:ffff8a23aec00000(0000) knlGS:0000000000000000
<4>[  ...] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
<4>[  ...] CR2: 000055b479d92068 CR3: 00000003764f6004 CR4: 00000000003606f0
<4>[  ...] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
<4>[  ...] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400

Have the client ignore the extra slashes in the server path when
mounting. This will also canonicalize the path, so that identical mounts
can be consilidated.

1) "//mydir1///mydir//"
2) "/mydir1/mydir"
3) "/mydir1/mydir/"

Regardless of the internal treatment of these paths, the kernel still
stores the original string including the leading '/' for presentation
to userland.

Signed-off-by: Xiubo Li <xiubli@redhat.com>
Signed-off-by: Jeff Layton <jlayton@kernel.org>