ceph: add support for encrypted snapshot names

Since filenames in encrypted directories are encrypted and shown as
a base64-encoded string when the directory is locked, make snapshot
names show a similar behaviour.

When creating a snapshot, .snap directories for every subdirectory will
show the snapshot name in the "long format":

  # mkdir .snap/my-snap
  # ls my-dir/.snap/
  _my-snap_1099511627782

Encrypted snapshots will need to be able to handle these by
encrypting/decrypting only the snapshot part of the string ('my-snap').

Also, since the MDS prevents snapshot names to be bigger than 240
characters it is necessary to adapt CEPH_NOHASH_NAME_MAX to accommodate
this extra limitation.

[ idryomov: drop const on !CONFIG_FS_ENCRYPTION branch too ]

Signed-off-by: Luís Henriques <lhenriques@suse.de>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Reviewed-by: Xiubo Li <xiubli@redhat.com>
Reviewed-by: Milind Changire <mchangir@redhat.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>