autofs4 - remove autofs4_lock

The autofs4_lock introduced by the rcu-walk changes has unnecessarily
broad scope. The locking is better handled by the per-autofs super
block lookup_lock.

Signed-off-by: Ian Kent <raven@themaw.net>
Acked-by: David Howells <dhowells@redhat.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>