nfsctl: switch to simple_recursive_removal()

	Use simple_recursive_removal() in nfsd_client_rmdir() rather than
open-coding it.  And use less heavy-handed locking to get from nfsctl
inode to its ->i_private...

Reviewed-by: Jeff Layton <jlayton@redhat.com>
Tested-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>