MIPS: Fix build error if __xchg() is not getting inlined.

If __xchg() is not getting inlined the outline version of the function
will have a reference to __xchg_called_with_bad_pointer() which does not
exist remaining.  Fixed by using BUILD_BUG_ON() to check for allowable
operand sizes.
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>
Patchwork: http://patchwork.linux-mips.org/patch/705/

arch/mips/include/asm/system.h

@@ -12,6 +12,7 @@
 #ifndef _ASM_SYSTEM_H
 #define _ASM_SYSTEM_H
 
+#include <linux/kernel.h>
 #include <linux/types.h>
 #include <linux/irqflags.h>
 
@@ -193,10 +194,6 @@ extern __u64 __xchg_u64_unsupported_on_32bit_kernels(volatile __u64 * m, __u64 v
 #define __xchg_u64 __xchg_u64_unsupported_on_32bit_kernels
 #endif
 
-/* This function doesn't exist, so you'll get a linker error
-   if something tries to do an invalid xchg().  */
-extern void __xchg_called_with_bad_pointer(void);
-
 static inline unsigned long __xchg(unsigned long x, volatile void * ptr, int size)
 {
 	switch (size) {
@@ -205,11 +202,17 @@ static inline unsigned long __xchg(unsigned long x, volatile void * ptr, int siz
 	case 8:
 		return __xchg_u64(ptr, x);
 	}
-	__xchg_called_with_bad_pointer();
+
 	return x;
 }
 
-#define xchg(ptr, x) ((__typeof__(*(ptr)))__xchg((unsigned long)(x), (ptr), sizeof(*(ptr))))
+#define xchg(ptr, x)							\
+({									\
+	BUILD_BUG_ON(sizeof(*(ptr)) & ~0xc);				\
+									\
+	((__typeof__(*(ptr)))						\
+		__xchg((unsigned long)(x), (ptr), sizeof(*(ptr))));	\
+})
 
 extern void set_handler(unsigned long offset, void *addr, unsigned long len);
 extern void set_uncached_handler(unsigned long offset, void *addr, unsigned long len);