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

99741bd 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 openzfs#16069

config/kernel-blk-queue.m4

@@ -377,13 +377,28 @@ AC_DEFUN([ZFS_AC_KERNEL_SRC_BLK_MQ], [
 		(void) blk_mq_alloc_tag_set(&tag_set);
 		return BLK_STS_OK;
 	], [])
+	ZFS_LINUX_TEST_SRC([blk_mq_rq_hctx], [
+		#include <linux/blk-mq.h>
+		#include <linux/blkdev.h>
+	], [
+		struct request rq = {0};
+		struct blk_mq_hw_ctx *hctx = NULL;
+		rq.mq_hctx = hctx;
+	], [])
 ])
 
 AC_DEFUN([ZFS_AC_KERNEL_BLK_MQ], [
 	AC_MSG_CHECKING([whether block multiqueue with blk_status_t is available])
 	ZFS_LINUX_TEST_RESULT([blk_mq], [
 		AC_MSG_RESULT(yes)
 		AC_DEFINE(HAVE_BLK_MQ, 1, [block multiqueue is available])
+		AC_MSG_CHECKING([whether block multiqueue hardware context is cached in struct request])
+		ZFS_LINUX_TEST_RESULT([blk_mq_rq_hctx], [
+			AC_MSG_RESULT(yes)
+			AC_DEFINE(HAVE_BLK_MQ_RQ_HCTX, 1, [block multiqueue hardware context is cached in struct request])
+		], [
+			AC_MSG_RESULT(no)
+		])
 	], [
 		AC_MSG_RESULT(no)
 	])

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);