signal: use BUILD_BUG() instead of _NSIG_WORDS_is_unsupported_size()

Kill _NSIG_WORDS_is_unsupported_size(), use BUILD_BUG() instead. This
simplifies the code, avoids the nested-externs warnings, and this way we
do not defer the problem to linker.

Also, fix the indentation in _SIG_SET_BINOP() and _SIG_SET_OP().

Note: this patch assumes that the code like "if (0) BUILD_BUG();" is
valid. If not (say __compiletime_error() is not defined and thus
__compiletime_error_fallback() uses a negative array) we should fix
BUILD_BUG() and/or BUILD_BUG_ON_MSG(). This code should be fine by
definition, this is the documented purpose of BUILD_BUG().

[[e-mail]: fix powerpc build failures]
Signed-off-by: Oleg Nesterov <[e-mail]>
Reported-by: Jeff Kirsher <[e-mail]>
Reviewed-by: Josh Triplett <[e-mail]>
Signed-off-by: Stephen Rothwell <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

include/linux/signal.h

···
 #define _LINUX_SIGNAL_H
 
 #include <linux/list.h>
+#include <linux/bug.h>
 #include <uapi/linux/signal.h>
 
 struct task_struct;
···
 
 static inline int sigisemptyset(sigset_t *set)
 {
-	extern void _NSIG_WORDS_is_unsupported_size(void);
 	switch (_NSIG_WORDS) {
 	case 4:
 		return (set->sig[3] | set->sig[2] |
···
 	case 1:
 		return set->sig[0] == 0;
 	default:
-		_NSIG_WORDS_is_unsupported_size();
+		BUILD_BUG();
 		return 0;
 	}
 }
···
 #define _SIG_SET_BINOP(name, op)					\
 static inline void name(sigset_t *r, const sigset_t *a, const sigset_t *b) \
 {									\
-	extern void _NSIG_WORDS_is_unsupported_size(void);		\
 	unsigned long a0, a1, a2, a3, b0, b1, b2, b3;			\
 									\
 	switch (_NSIG_WORDS) {						\
-	    case 4:							\
+	case 4:								\
 		a3 = a->sig[3]; a2 = a->sig[2];				\
 		b3 = b->sig[3]; b2 = b->sig[2];				\
 		r->sig[3] = op(a3, b3);					\
 		r->sig[2] = op(a2, b2);					\
-	    case 2:							\
+	case 2:								\
 		a1 = a->sig[1]; b1 = b->sig[1];				\
 		r->sig[1] = op(a1, b1);					\
-	    case 1:							\
+	case 1:								\
 		a0 = a->sig[0]; b0 = b->sig[0];				\
 		r->sig[0] = op(a0, b0);					\
 		break;							\
-	    default:							\
-		_NSIG_WORDS_is_unsupported_size();			\
+	default:							\
+		BUILD_BUG();						\
 	}								\
 }
 
···
 #define _SIG_SET_OP(name, op)						\
 static inline void name(sigset_t *set)					\
 {									\
-	extern void _NSIG_WORDS_is_unsupported_size(void);		\
-									\
 	switch (_NSIG_WORDS) {						\
-	    case 4: set->sig[3] = op(set->sig[3]);			\
-		    set->sig[2] = op(set->sig[2]);			\
-	    case 2: set->sig[1] = op(set->sig[1]);			\
-	    case 1: set->sig[0] = op(set->sig[0]);			\
+	case 4:	set->sig[3] = op(set->sig[3]);				\
+		set->sig[2] = op(set->sig[2]);				\
+	case 2:	set->sig[1] = op(set->sig[1]);				\
+	case 1:	set->sig[0] = op(set->sig[0]);				\
 		    break;						\
-	    default:							\
-		_NSIG_WORDS_is_unsupported_size();			\
+	default:							\
+		BUILD_BUG();						\
 	}								\
 }