kernel: Fix sparse warning for ACCESS_ONCE

Commit 927609d622a3 ("kernel: tighten rules for ACCESS ONCE") results in
sparse warnings like "Using plain integer as NULL pointer" - Let's add a
type cast to the dummy assignment.
To avoid warnings lik "sparse: warning: cast to restricted __hc32" we also
use __force on that cast.

Fixes: 927609d622a3 ("kernel: tighten rules for ACCESS ONCE")
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>

include/linux/compiler.h

@@ -461,7 +461,7 @@ static __always_inline void __assign_once_size(volatile void *p, void *res, int
  * If possible use READ_ONCE/ASSIGN_ONCE instead.
  */
 #define __ACCESS_ONCE(x) ({ \
-	 __maybe_unused typeof(x) __var = 0; \
+	 __maybe_unused typeof(x) __var = (__force typeof(x)) 0; \
 	(volatile typeof(x) *)&(x); })
 #define ACCESS_ONCE(x) (*__ACCESS_ONCE(x))