dcache: fix outdated DCACHE_NEED_LOOKUP comment

The DCACHE_NEED_LOOKUP case referred to here was removed with
39e3c955 "vfs: remove
DCACHE_NEED_LOOKUP".

There are only four real_lookup() callers and all of them pass in an
unhashed dentry just returned from d_alloc.

Signed-off-by: J. Bruce Fields <bfields@redhat.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>