powerpc: Fix BUG_ON() reporting in real mode

I ran into this issue while debugging an early boot problem. The system
hit a BUG_ON() but report bug failed to print the line number and file
name. The reason being that the system was running in real mode and
report_bug() searches for addresses in the PAGE_OFFSET+ region.
Suggested-by: Paul Mackerras <paulus@samba.org>
Signed-off-by: Balbir Singh <bsingharora@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/kernel/traps.c

@@ -1148,6 +1148,7 @@ void __kprobes program_check_exception(struct pt_regs *regs)
 		goto bail;
 	}
 	if (reason & REASON_TRAP) {
+		unsigned long bugaddr;
 		/* Debugger is first in line to stop recursive faults in
 		 * rcu_lock, notify_die, or atomic_notifier_call_chain */
 		if (debugger_bpt(regs))
@@ -1158,8 +1159,15 @@ void __kprobes program_check_exception(struct pt_regs *regs)
 				== NOTIFY_STOP)
 			goto bail;
 
+		bugaddr = regs->nip;
+		/*
+		 * Fixup bugaddr for BUG_ON() in real mode
+		 */
+		if (!is_kernel_addr(bugaddr) && !(regs->msr & MSR_IR))
+			bugaddr += PAGE_OFFSET;
+
 		if (!(regs->msr & MSR_PR) &&  /* not user-mode */
-		    report_bug(regs->nip, regs) == BUG_TRAP_TYPE_WARN) {
+		    report_bug(bugaddr, regs) == BUG_TRAP_TYPE_WARN) {
 			regs->nip += 4;
 			goto bail;
 		}