mm: fix CONFIG_DEBUG_VM_RB description

This appears to be a copy/paste error.  Update the description to
reflect extra rbtree debug and checks for the config option instead of
duplicating CONFIG_DEBUG_VM.

Signed-off-by: Davidlohr Bueso <davidlohr@hp.com>
Cc: Aswin Chandramouleeswaran <aswin@hp.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

lib/Kconfig.debug

@@ -505,8 +505,7 @@ config DEBUG_VM_RB
 	bool "Debug VM red-black trees"
 	depends on DEBUG_VM
 	help
-	  Enable this to turn on more extended checks in the virtual-memory
-	  system that may impact performance.
+	  Enable VM red-black tree debugging information and extra validations.
 
 	  If unsure, say N.