btrfs: handle errors in btrfs_reloc_clone_csums properly

In the cow path we will clone the reloc csums for relocated data
extents, and if there's an error we already have an ordered extent and
rely on the ordered extent finishing to clean everything up.

There's a problem however, we don't mark the ordered extent with an
error, we pretend like everything was just fine.  If we were at the end
of our range we won't actually bubble up this error anywhere, and we
could end up inserting an extent that doesn't have csums where it should
have them.

Fix this by adding a helper to mark the ordered extent with an error,
and then use this when we fail to lookup the csums in
btrfs_reloc_clone_csums.  Use this helper in the other place where we
use the same pattern while we're here.

This will prevent us from erroneously inserting the extent that doesn't
have the required checksums.

Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: David Sterba <dsterba@suse.com>