zvol_os: fix compile with blk-mq on Linux 4.x

99741bde5 accesses a cached blk-mq hardware context through the mq_hctx
field of struct request. However, this field did not exist until 5.0.
Before that, the private function blk_mq_map_queue() was used to dig it
out of broader queue context. This commit detects this situation, and
handles it with a poor-man's simulation of that function.

Sponsored-by: Klara, Inc.
Sponsored-by: Wasabi Technology, Inc.
Reviewed-by: Ameer Hamza <ahamza@ixsystems.com>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Rob Norris <rob.norris@klarasystems.com>
Closes #16069

module/os/linux/zfs/zvol_os.c

@@ -551,7 +551,12 @@ zvol_request_impl(zvol_state_t *zv, struct bio *bio, struct request *rq,
 	uint_t taskq_hash;
 #ifdef HAVE_BLK_MQ
 	if (rq)
+#ifdef HAVE_BLK_MQ_RQ_HCTX
 		blk_mq_hw_queue = rq->mq_hctx->queue_num;
+#else
+		blk_mq_hw_queue =
+		    rq->q->queue_hw_ctx[rq->q->mq_map[rq->cpu]]->queue_num;
+#endif
 #endif
 	taskq_hash = cityhash4((uintptr_t)zv, offset >> ZVOL_TASKQ_OFFSET_SHIFT,
 	    blk_mq_hw_queue, 0);