x86/fault: Reword initial BUG message for unhandled page faults

Reword the NULL pointer dereference case to simply state that a NULL
pointer was dereferenced, i.e. drop "unable to handle" as that implies
that there are instances where the kernel actual does handle NULL
pointer dereferences, which is not true barring funky exception fixup.

For the non-NULL case, replace "kernel paging request" with "page fault"
as the kernel can technically oops on faults that originated in user
code.  Dropping "kernel" also allows future patches to provide detailed
information on where the fault occurred, e.g. user vs. kernel, without
conflicting with the initial BUG message.

In both cases, replace "at address=" with wording more appropriate to
the oops, as "at" may be interpreted as stating that the address is the
RIP of the instruction that faulted.

Last, and probably least, further qualify the NULL-pointer path by
checking that the fault actually originated in kernel code.  It's
technically possible for userspace to map address 0, and not printing
a super specific message is the least of our worries if the kernel does
manage to oops on an actual NULL pointer dereference from userspace.

Before:
    BUG: unable to handle kernel NULL pointer dereference at ffffbeef00000000
    BUG: unable to handle kernel paging request at ffffbeef00000000

After:
    BUG: kernel NULL pointer dereference, address = 0000000000000008
    BUG: unable to handle page fault for address = ffffbeef00000000
Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Rik van Riel <riel@surriel.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Yu-cheng Yu <yu-cheng.yu@intel.com>
Link: http://lkml.kernel.org/r/20181221213657.27628-2-sean.j.christopherson@intel.comSigned-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/mm/fault.c

@@ -644,9 +644,12 @@ show_fault_oops(struct pt_regs *regs, unsigned long error_code, unsigned long ad
 				from_kuid(&init_user_ns, current_uid()));
 	}
 
-	pr_alert("BUG: unable to handle kernel %s at %px\n",
-		 address < PAGE_SIZE ? "NULL pointer dereference" : "paging request",
-		 (void *)address);
+	if (address < PAGE_SIZE && !user_mode(regs))
+		pr_alert("BUG: kernel NULL pointer dereference, address = %px\n",
+			(void *)address);
+	else
+		pr_alert("BUG: unable to handle page fault for address = %px\n",
+			(void *)address);
 
 	err_txt[0] = 0;