blk-mq: make the blk-mq stacking code optional

The code to stack blk-mq drivers is only used by dm-multipath, and
will preferably stay that way.  Make it optional and only selected
by device mapper, so that the buildbots more easily catch abuses
like the one that slipped in in the ufs driver in the last merged
window.  Another positive side effects is that kernel builds without
device mapper shrink a little bit as well.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Mike Snitzer <snitzer@redhat.com>
Link: https://lore.kernel.org/r/20220215100540.3892965-2-hch@lst.de
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/Kconfig

@@ -230,6 +230,9 @@ config BLK_PM
 config BLOCK_HOLDER_DEPRECATED
 	bool
 
+config BLK_MQ_STACKING
+	bool
+
 source "block/Kconfig.iosched"
 
 endif # BLOCK

block/blk-mq.c

@@ -2840,6 +2840,7 @@ void blk_mq_submit_bio(struct bio *bio)
 				blk_mq_try_issue_directly(rq->mq_hctx, rq));
 }
 
+#ifdef CONFIG_BLK_MQ_STACKING
 /**
  * blk_cloned_rq_check_limits - Helper function to check a cloned request
  *                              for the new queue limits
@@ -3017,6 +3018,7 @@ free_and_out:
 	return -ENOMEM;
 }
 EXPORT_SYMBOL_GPL(blk_rq_prep_clone);
+#endif /* CONFIG_BLK_MQ_STACKING */
 
 /*
  * Steal bios from a request and add them to a bio list.

drivers/md/Kconfig

@@ -204,6 +204,7 @@ config BLK_DEV_DM
 	tristate "Device mapper support"
 	select BLOCK_HOLDER_DEPRECATED if SYSFS
 	select BLK_DEV_DM_BUILTIN
+	select BLK_MQ_STACKING
 	depends on DAX || DAX=n
 	help
 	  Device-mapper is a low level volume manager.  It works by allowing