vfs: uninline full_name_hash()

.. and also use it in lookup_one_len() rather than open-coding it.

There aren't any performance-critical users, so inlining it is silly.
But it wouldn't matter if it wasn't for the fact that the word-at-a-time
dentry name patches want to conditionally replace the function, and
uninlining it sets the stage for that.

So again, this is a preparatory patch that doesn't change any semantics,
and only prepares for a much cleaner and testable word-at-a-time dentry
name accessor patch.

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