Revert "vfs: add lookup_hash() helper"

Revert "vfs: add lookup_hash() helper"

This reverts commit 3c9fe8cdff1b889a059a30d22f130372f2b3885f.

As Miklos points out in commit c1b2cc1a765a, the "lookup_hash()" helper
is now unused, and in fact, with the hash salting changes, since the
hash of a dentry name now depends on the directory dentry it is in, the
helper function isn't even really likely to be useful.

So rather than keep it around in case somebody else might end up finding
a use for it, let's just remove the helper and not trick people into
thinking it might be a useful thing.

For example, I had obviously completely missed how the helper didn't
follow the normal dentry hashing patterns, and how the hash salting
patch broke overlayfs.  Things would quietly build and look sane, but
not work.

Suggested-by: Miklos Szeredi <mszeredi@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>