core: Always sort incoming xattrs

core: Always sort incoming xattrs

When recomputing selinux attrs during commit, we weren't sorting,
which could cause various issues like fsck failures.

This is a big hammer; change things so we always canonicalize
(i.e. sort) the incoming xattrs when creating a file header
and directory metadata.

I think almost all places in the code were already keeping
things sorted, but it's better to ensure correctness first.
If we ever have some performance issue (I'm doubtful) we
could add something like `_ostree_file_header_known_canonicalized`
or so.

Closes: https://github.com/ostreedev/ostree/issues/3343
Signed-off-by: Colin Walters <walters@verbum.org>