nfsd: fsnotify on rmdir under nfsd/clients/

Userspace should be able to monitor nfsd/clients/ to see when clients
come and go, but we're failing to send fsnotify events.

Cc: stable@kernel.org
Signed-off-by: J. Bruce Fields <bfields@redhat.com>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>