vfs: link_path_walk: clarify and improve name hashing interface

vfs: link_path_walk: clarify and improve name hashing interface

Now that we clearly only care about the length of the name we just
parsed, we can simplify and clarify the interface to "name_hash()", and
move the actual nd->last field setting in there.

That makes everything simpler, and this way don't mix the hash and the
length together only to then immediately unmix them again.

We still eventually want the combined mixed "hashlen" for when we look
things up in the dentry cache, but inside link_path_walk() it's simpler
and clearer to just deal with the path component length.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>