block: add BIG FAT WARNING to CONFIG_DEBUG_BLOCK_EXT_DEVT

CONFIG_DEBUG_BLOCK_EXT_DEVT can break booting even on some modern
distros.  Add BIG FAT WARNING to keep people at a safe distance.
Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>

lib/Kconfig.debug

@@ -652,6 +652,11 @@ config DEBUG_BLOCK_EXT_DEVT
 	depends on BLOCK
 	default n
 	help
+	  BIG FAT WARNING: ENABLING THIS OPTION MIGHT BREAK BOOTING ON
+	  SOME DISTRIBUTIONS.  DO NOT ENABLE THIS UNLESS YOU KNOW WHAT
+	  YOU ARE DOING.  Distros, please enable this and fix whatever
+	  is broken.
+
 	  Conventionally, block device numbers are allocated from
 	  predetermined contiguous area.  However, extended block area
 	  may introduce non-contiguous block device numbers.  This