x86: add might_sleep() to do_page_fault()

Impact: widen debug checks

VirtualBox calls do_page_fault() from an atomic context but runs into a
might_sleep() way pas this point, cure that.
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

arch/x86/mm/fault.c

@@ -888,6 +888,12 @@ void __kprobes do_page_fault(struct pt_regs *regs, unsigned long error_code)
 			return;
 		}
 		down_read(&mm->mmap_sem);
+	} else {
+		/*
+		 * The above down_read_trylock() might have succeeded in which
+		 * case we'll have missed the might_sleep() from down_read().
+		 */
+		might_sleep();
 	}
 
 	vma = find_vma(mm, address);