compiler.h: Enforce that READ_ONCE_NOCHECK() access size is sizeof(long)

READ_ONCE_NOCHECK() unconditionally performs a sizeof(long)-sized access,
so enforce that the size of the pointed-to object that we are loading
from is the same size as 'long'.

Reported-by: Marco Elver <elver@google.com>
Signed-off-by: Will Deacon <will@kernel.org>

include/linux/compiler.h

@@ -254,9 +254,12 @@ unsigned long __read_once_word_nocheck(const void *addr)
  */
 #define READ_ONCE_NOCHECK(x)						\
 ({									\
-	unsigned long __x = __read_once_word_nocheck(&(x));		\
+	unsigned long __x;						\
+	compiletime_assert(sizeof(x) == sizeof(__x),			\
+		"Unsupported access size for READ_ONCE_NOCHECK().");	\
+	__x = __read_once_word_nocheck(&(x));				\
 	smp_read_barrier_depends();					\
-	__x;								\
+	(typeof(x))__x;							\
 })
 
 static __no_kasan_or_inline