client: use the fs' full path instead of from mountpoint's root

client: use the fs' full path instead of from mountpoint's root

The mountpoint's root ino# possibly not be the full CephFS
filesystem root, it's just the mountpoint of this particular client.
Just prepend the mountpoint path to the full path.

Introduced-by: c1bf8d88e9d client: check the cephx mds auth access for setattr
Introduced-by: ce216595c03 client: check the cephx mds auth access for open
Fixes: https://github.com/ceph/ceph/pull/48027#issuecomment-1741019086
Signed-off-by: Xiubo Li <xiubli@redhat.com>