powerpc/powernv/mce: Don't silently restart the machine

On MCE the current code will restart the machine with
ppc_md.restart(). This case was extremely unlikely since
prior to that a skiboot call is made and that resulted in
a checkstop for analysis.

With newer skiboots, on P9 we don't checkstop the box by
default, instead we return back to the kernel to extract
useful information at the time of the MCE. While we still
get this information, this patch converts the restart to
a panic(), so that if configured a dump can be taken and
we can track and probably debug the potential issue causing
the MCE.

Signed-off-by: Balbir Singh <bsingharora@gmail.com>
Reviewed-by: Nicholas Piggin <npiggin@gmail.com>
Reviewed-by: Stewart Smith <stewart@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/platforms/powernv/opal.c

@@ -490,9 +490,12 @@ void pnv_platform_error_reboot(struct pt_regs *regs, const char *msg)
 	 *    opal to trigger checkstop explicitly for error analysis.
 	 *    The FSP PRD component would have already got notified
 	 *    about this error through other channels.
+	 * 4. We are running on a newer skiboot that by default does
+	 *    not cause a checkstop, drops us back to the kernel to
+	 *    extract context and state at the time of the error.
 	 */
 
-	ppc_md.restart(NULL);
+	panic(msg);
 }
 
 int opal_machine_check(struct pt_regs *regs)