sparse: define __must_be_array() for __CHECKER__

commit c5e631cf ("ARRAY_SIZE: check for type") added __must_be_array().
But sparse can't parse this gcc extention.

Now make C=2 makes following sparse errors a lot.

  kernel/futex.c:2699:25: error: No right hand side of '+'-expression

Because __must_be_array() is used for ARRAY_SIZE() macro and it is
used very widely.

This patch fixes it.
Signed-off-by: NKOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Cc: Dave Hansen <dave@linux.vnet.ibm.com>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

include/linux/compiler-gcc.h

@@ -34,8 +34,12 @@
     __asm__ ("" : "=r"(__ptr) : "0"(ptr));		\
     (typeof(ptr)) (__ptr + (off)); })
 
+#ifdef __CHECKER__
+#define __must_be_array(arr) 0
+#else
 /* &a[0] degrades to a pointer: a different type from an array */
 #define __must_be_array(a) BUILD_BUG_ON_ZERO(__same_type((a), &(a)[0]))
+#endif
 
 /*
  * Force always-inline if the user requests it so via the .config,