x86/mm: Convert unreachable() to BUG()

Commit 2190966 ("x86: Convert unreachable() to BUG()") missed
one.

And after commit 06e2474 ("objtool: Remove
annotate_{,un}reachable()") the invalid use of unreachable()
(rightfully) triggers warnings:

  vmlinux.o: warning: objtool: page_fault_oops() falls through to next function is_prefetch()

Fixes: 2190966 ("x86: Convert unreachable() to BUG()")
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20241216093215.GD12338@noisy.programming.kicks-ass.net

arch/x86/mm/fault.c

@@ -678,7 +678,7 @@ page_fault_oops(struct pt_regs *regs, unsigned long error_code,
 			      ASM_CALL_ARG3,
 			      , [arg1] "r" (regs), [arg2] "r" (address), [arg3] "r" (&info));
 
-		unreachable();
+		BUG();
 	}
 #endif