dm: fix write same requests counting

When processing write same requests, fix dm to send the configured
number of WRITE SAME requests to the target rather than the number of
discards, which is not always the same.

Device-mapper WRITE SAME support was introduced by commit
23508a96cd2e857d57044a2ed7d305f2d9daf441 ("dm: add WRITE SAME support").

Signed-off-by: Alasdair G Kergon <[e-mail]>
Acked-by: Mike Snitzer <[e-mail]>

drivers/md/dm.c

···
 {
 	struct dm_target *ti;
 	sector_t len;
+	unsigned num_requests;
 
 	do {
 		ti = dm_table_find_target(ci->map, ci->sector);
···
 		 * reconfiguration might also have changed that since the
 		 * check was performed.
 		 */
-		if (!get_num_requests || !get_num_requests(ti))
+		num_requests = get_num_requests ? get_num_requests(ti) : 0;
+		if (!num_requests)
 			return -EOPNOTSUPP;
 
 		if (is_split_required && !is_split_required(ti))
···
 		else
 			len = min(ci->sector_count, max_io_len(ci->sector, ti));
 
-		__issue_target_requests(ci, ti, ti->num_discard_requests, len);
+		__issue_target_requests(ci, ti, num_requests, len);
 
 		ci->sector += len;
 	} while (ci->sector_count -= len);