btrfs: switch args for comp_*_refs

Make it more consistent, we want the inserted ref to be compared against
what's already in there.  This will make the order go from lowest seq ->
highest seq, which will make us more likely to make forward progress if
there's a seqlock currently held.

Signed-off-by: Josef Bacik <jbacik@fb.com>
Signed-off-by: David Sterba <dsterba@suse.com>