vdso: print fatal signals

Add the print-fatal-signals=1 boot option and the
/proc/sys/kernel/print-fatal-signals runtime switch.

This feature prints some minimal information about userspace segfaults to
the kernel console.  This is useful to find early bootup bugs where
userspace debugging is very hard.

Defaults to off.

[akpm@linux-foundation.org: Don't add new sysctl numbers]
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Arjan van de Ven <arjan@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
index 182c6a3..ab38322 100644
--- a/Documentation/kernel-parameters.txt
+++ b/Documentation/kernel-parameters.txt
@@ -1383,6 +1383,12 @@
 			autoconfiguration.
 			Ranges are in pairs (memory base and size).
 
+	print-fatal-signals=
+			[KNL] debug: print fatal signals
+			print-fatal-signals=1: print segfault info to
+			the kernel console.
+			default: off.
+
 	profile=	[KNL] Enable kernel profiling via /proc/profile
 			Format: [schedule,]<number>
 			Param: "schedule" - profile schedule points.