ceph: do not release the global snaprealm until unmounting

The global snaprealm would be created and then destroyed immediately
every time when updating it.

URL: https://tracker.ceph.com/issues/54362


Signed-off-by: Xiubo Li <xiubli@redhat.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>