autofs4 - fix dentry leak in autofs4_expire_direct()

There is a missing dput() when returning from autofs4_expire_direct()
when we see that the dentry is already a pending mount.

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>