powerpc: Fix the corrupt r3 error during MCE handling.

During Machine Check interrupt on pseries platform, R3 generally points to
memory region inside RTAS (FWNMI) area. We see r3 corruption because when RTAS
delivers the machine check exception it passes the address inside FWNMI area
with the top most bit set. This patch fixes this issue by masking top two bit
in machine check exception handler.
Signed-off-by: Mahesh Salgaonkar <mahesh@linux.vnet.ibm.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

arch/powerpc/platforms/pseries/ras.c

@@ -287,6 +287,9 @@ static struct rtas_error_log *fwnmi_get_errinfo(struct pt_regs *regs)
 	unsigned long *savep;
 	struct rtas_error_log *h, *errhdr = NULL;
 
+	/* Mask top two bits */
+	regs->gpr[3] &= ~(0x3UL << 62);
+
 	if (!VALID_FWNMI_BUFFER(regs->gpr[3])) {
 		printk(KERN_ERR "FWNMI: corrupt r3 0x%016lx\n", regs->gpr[3]);
 		return NULL;