lookup_one_len_unlocked(): use lookup_dcache()

No need to lock parent just because of ->d_revalidate() on child;
contrary to the stale comment, lookup_dcache() *can* be used without
locking the parent.  Result can be moved as soon as we return, of
course, but the same is true for lookup_one_len_unlocked() itself.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>