kexec: sysrq: simplify sysrq-c handler

Currently the sysrq-c handler is bit over-engineered.  Its behavior is
dependent on a few compile time and run time factors that alter its
behavior which is really unnecessecary.

If CONFIG_KEXEC is not configured, sysrq-c, crashes the system with a NULL
pointer dereference.  If CONFIG_KEXEC is configured, it calls crash_kexec
directly, which implies that the kexec kernel will either be booted (if
its been previously loaded), or it will simply do nothing (the no kexec
kernel has been loaded).

It would be much easier to just simplify the whole thing to dereference a
NULL pointer all the time regardless of configuration.  That way, it will
always try to crash the system, and if a kexec kernel has been loaded into
reserved space, it will still boot from the page fault trap handler
(assuming panic_on_oops is set appropriately).

[akpm@linux-foundation.org: build fix]
Signed-off-by: Neil Horman <nhorman@tuxdriver.com>
Acked-by: Vivek Goyal <vgoyal@redhat.com>
Cc: Brayan Arraes <brayan@yack.com.br>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/char/sysrq.c

@@ -121,20 +121,17 @@ static struct sysrq_key_op sysrq_unraw_op = {
 #define sysrq_unraw_op (*(struct sysrq_key_op *)0)
 #endif /* CONFIG_VT */
 
-#ifdef CONFIG_KEXEC
-static void sysrq_handle_crashdump(int key, struct tty_struct *tty)
+static void sysrq_handle_crash(int key, struct tty_struct *tty)
 {
-	crash_kexec(get_irq_regs());
+	char *killer = NULL;
+	*killer = 1;
 }
 static struct sysrq_key_op sysrq_crashdump_op = {
-	.handler	= sysrq_handle_crashdump,
-	.help_msg	= "Crashdump",
-	.action_msg	= "Trigger a crashdump",
+	.handler	= sysrq_handle_crash,
+	.help_msg	= "Crash",
+	.action_msg	= "Trigger a crash",
 	.enable_mask	= SYSRQ_ENABLE_DUMP,
 };
-#else
-#define sysrq_crashdump_op (*(struct sysrq_key_op *)0)
-#endif
 
 static void sysrq_handle_reboot(int key, struct tty_struct *tty)
 {