ceph: new device mount syntax

ceph: new device mount syntax

Old mount device syntax (source) has the following problems:

- mounts to the same cluster but with different fsnames
  and/or creds have identical device string which can
  confuse xfstests.

- Userspace mount helper tool resolves monitor addresses
  and fill in mon addrs automatically, but that means the
  device shown in /proc/mounts is different than what was
  used for mounting.

New device syntax is as follows:

  cephuser@fsid.mycephfs2=/path

Note, there is no "monitor address" in the device string.
That gets passed in as mount option. This keeps the device
string same when monitor addresses change (on remounts).

Also note that the userspace mount helper tool is backward
compatible. I.e., the mount helper will fallback to using
old syntax after trying to mount with the new syntax.

Signed-off-by: Venky Shankar <vshankar@redhat.com>
Signed-off-by: Jeff Layton <jlayton@kernel.org>