vfs: don't revalidate just looked up dentry

__lookup_hash() calls ->lookup() if the dentry needs lookup and on success
revalidates the dentry (all under dir->i_mutex).

While this is harmless it doesn't make a lot of sense.

Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>