ceph: prevent snapshot creation in encrypted locked directories

With snapshot names encryption we can not allow snapshots to be created in
locked directories because the names wouldn't be encrypted.  This patch
forces the directory to be unlocked to allow a snapshot to be created.

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>