btrfs: avoid unnecessary bio dereference at run_one_async_done()

We have dereferenced the async_submit_bio structure and extracted the bio
pointer into a local variable, so there's no need to dereference it again
when calling btrfs_bio_end_io(). Just use "bio->bi_status" instead of the
longer expression "async->bbio->bio.bi_status".

Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>