block: Consolidate command flag and queue limit checks for merges

 - blk_check_merge_flags() verifies that cmd_flags / bi_rw are
   compatible. This function is called for both req-req and req-bio
   merging.

 - blk_rq_get_max_sectors() and blk_queue_get_max_sectors() can be used
   to query the maximum sector count for a given request or queue. The
   calls will return the right value from the queue limits given the
   type of command (RW, discard, write same, etc.)

Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>
Acked-by: Mike Snitzer <snitzer@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
diff --git a/block/blk-core.c b/block/blk-core.c
index 5cc2929..33eded0 100644
--- a/block/blk-core.c
+++ b/block/blk-core.c
@@ -1866,8 +1866,7 @@
 	if (!rq_mergeable(rq))
 		return 0;
 
-	if (blk_rq_sectors(rq) > queue_max_sectors(q) ||
-	    blk_rq_bytes(rq) > queue_max_hw_sectors(q) << 9) {
+	if (blk_rq_sectors(rq) > blk_queue_get_max_sectors(q, rq->cmd_flags)) {
 		printk(KERN_ERR "%s: over max size limit.\n", __func__);
 		return -EIO;
 	}