blk-mq: add BLK_MQ_F_DEFER_ISSUE support flag

Drivers can now tell blk-mq if they take advantage of the deferred
issue through 'last' or not. If they do, don't do queue-direct
for sync IO. This is a preparation patch for the nvme conversion.

Signed-off-by: Jens Axboe <axboe@fb.com>

include/linux/blk-mq.h

@@ -146,6 +146,7 @@ enum {
 	BLK_MQ_F_TAG_SHARED	= 1 << 1,
 	BLK_MQ_F_SG_MERGE	= 1 << 2,
 	BLK_MQ_F_SYSFS_UP	= 1 << 3,
+	BLK_MQ_F_DEFER_ISSUE	= 1 << 4,
 
 	BLK_MQ_S_STOPPED	= 0,
 	BLK_MQ_S_TAG_ACTIVE	= 1,