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: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Cc: Dave Hansen <dave@linux.vnet.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>