powerpc/mm: Make NULL pointer deferences explicit on bad page faults.

As several other arches including x86, this patch makes it explicit
that a bad page fault is a NULL pointer dereference when the fault
address is lower than PAGE_SIZE

In the mean time, this page makes all bad_page_fault() messages
shorter so that they remain on one single line. And it prefixes them
by "BUG: " so that they get easily grepped.
Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
[mpe: Avoid pr_cont()]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/mm/fault.c

@@ -650,21 +650,22 @@ void bad_page_fault(struct pt_regs *regs, unsigned long address, int sig)
 	switch (TRAP(regs)) {
 	case 0x300:
 	case 0x380:
-		printk(KERN_ALERT "Unable to handle kernel paging request for "
-			"data at address 0x%08lx\n", regs->dar);
+		pr_alert("BUG: %s at 0x%08lx\n",
+			 regs->dar < PAGE_SIZE ? "Kernel NULL pointer dereference" :
+			 "Unable to handle kernel data access", regs->dar);
 		break;
 	case 0x400:
 	case 0x480:
-		printk(KERN_ALERT "Unable to handle kernel paging request for "
-			"instruction fetch\n");
+		pr_alert("BUG: Unable to handle kernel instruction fetch%s",
+			 regs->nip < PAGE_SIZE ? " (NULL pointer?)\n" : "\n");
 		break;
 	case 0x600:
-		printk(KERN_ALERT "Unable to handle kernel paging request for "
-			"unaligned access at address 0x%08lx\n", regs->dar);
+		pr_alert("BUG: Unable to handle kernel unaligned access at 0x%08lx\n",
+			 regs->dar);
 		break;
 	default:
-		printk(KERN_ALERT "Unable to handle kernel paging request for "
-			"unknown fault\n");
+		pr_alert("BUG: Unable to handle unknown paging fault at 0x%08lx\n",
+			 regs->dar);
 		break;
 	}
 	printk(KERN_ALERT "Faulting instruction address: 0x%08lx\n",