dm: test for REQ_ATOMIC in dm_accept_partial_bio()

Any bio with REQ_ATOMIC flag set should never be split or partially
completed, so BUG_ON() on this scenario in dm_accept_partial_bio() (whose
intent is to allow partial completions).

Also, we must reject atomic bio to targets that don't support them,
otherwise this BUG could be triggered by stray bios that have the
REQ_ATOMIC set.

Signed-off-by: John Garry <[e-mail]>
Signed-off-by: Mikulas Patocka <[e-mail]>
Tested-by: John Garry <[e-mail]>

drivers/md/dm.c

···
 	BUG_ON(dm_tio_flagged(tio, DM_TIO_IS_DUPLICATE_BIO));
 	BUG_ON(bio_sectors > *tio->len_ptr);
 	BUG_ON(n_sectors > bio_sectors);
+	BUG_ON(bio->bi_opf & REQ_ATOMIC);
 
 	if (static_branch_unlikely(&zoned_enabled) &&
 	    unlikely(bdev_is_zoned(bio->bi_bdev))) {
···
 	ci->submit_as_polled = !!(ci->bio->bi_opf & REQ_POLLED);
 
 	len = min_t(sector_t, max_io_len(ti, ci->sector), ci->sector_count);
-	if (ci->bio->bi_opf & REQ_ATOMIC && len != ci->sector_count)
-		return BLK_STS_IOERR;
+	if (ci->bio->bi_opf & REQ_ATOMIC) {
+		if (unlikely(!dm_target_supports_atomic_writes(ti->type)))
+			return BLK_STS_IOERR;
+		if (unlikely(len != ci->sector_count))
+			return BLK_STS_IOERR;
+	}
 
 	setup_split_accounting(ci, len);