blk-mq: fix debugfs compilation issues

This fixes a couple of problems:

1. In the !CONFIG_DEBUG_FS case, the stub definitions were bogus.
2. In the !CONFIG_BLOCK case, blk-mq-debugfs.c shouldn't be compiled at
   all.

Fix the stub definitions and add a CONFIG_BLK_DEBUG_FS Kconfig option.

Fixes: 07e4fead45e6 ("blk-mq: create debugfs directory tree")
Signed-off-by: Omar Sandoval <osandov@fb.com>

Augment Kconfig description.

Signed-off-by: Jens Axboe <axboe@fb.com>
diff --git a/block/Makefile b/block/Makefile
index 6cabe6b..317165f 100644
--- a/block/Makefile
+++ b/block/Makefile
@@ -26,4 +26,4 @@
 obj-$(CONFIG_BLK_MQ_PCI)	+= blk-mq-pci.o
 obj-$(CONFIG_BLK_DEV_ZONED)	+= blk-zoned.o
 obj-$(CONFIG_BLK_WBT)		+= blk-wbt.o
-obj-$(CONFIG_DEBUG_FS)		+= blk-mq-debugfs.o
+obj-$(CONFIG_BLK_DEBUG_FS)	+= blk-mq-debugfs.o