include/linux/kernel.h: make might_fault() a nop for !MMU

The machine cannot fault if !MUU, so make might_fault() a nop for !MMU.

This fixes below build error if
!CONFIG_MMU && (CONFIG_PROVE_LOCKING=y || CONFIG_DEBUG_ATOMIC_SLEEP=y):

  arch/arm/kernel/built-in.o: In function `arch_ptrace':
  arch/arm/kernel/ptrace.c:852: undefined reference to `might_fault'
  arch/arm/kernel/built-in.o: In function `restore_sigframe':
  arch/arm/kernel/signal.c:173: undefined reference to `might_fault'
  ...
  arch/arm/kernel/built-in.o:arch/arm/kernel/signal.c:177: more undefined references to `might_fault' follow
  make: *** [vmlinux] Error 1
Signed-off-by: Axel Lin <axel.lin@ingics.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/kernel.h

@@ -193,7 +193,8 @@ extern int _cond_resched(void);
 		(__x < 0) ? -__x : __x;		\
 	})
 
-#if defined(CONFIG_PROVE_LOCKING) || defined(CONFIG_DEBUG_ATOMIC_SLEEP)
+#if defined(CONFIG_MMU) && \
+	(defined(CONFIG_PROVE_LOCKING) || defined(CONFIG_DEBUG_ATOMIC_SLEEP))
 void might_fault(void);
 #else
 static inline void might_fault(void) { }