mac: rework labelling code to be simpler, and less racy

mac: rework labelling code to be simpler, and less racy

This merges the various labelling calls into a single label_fix_full(),
which can operate on paths, on inode fds, and in a dirfd/fname style
(i.e. like openat()). It also systematically separates the path to look
up in the db from the path we actually use to reference the inode to
relabel.

This then ports tmpfiles over to labelling by fd. This should make the
code a bit less racy, as we'll try hard to always operate on the very
same inode, pinning it via an fd.

User-visibly the behaviour should not change.