s390: Mark psw in __load_psw_mask() as __unitialized

Without __unitialized, the following code is generated when
INIT_STACK_ALL_ZERO is enabled:

86: d7 0f f0 a0 f0 a0     xc      160(16,%r15), 160(%r15)
8c: e3 40 f0 a0 00 24     stg     %r4, 160(%r15)
92: c0 10 00 00 00 08     larl    %r1, 0xa2
98: e3 10 f0 a8 00 24     stg     %r1, 168(%r15)
9e: b2 b2 f0 a0           lpswe   160(%r15)

The xc is not adding any security because psw is fully initialized
with the following instructions. Add __unitialized to the psw
definitiation to avoid the superfluous clearing of psw.
Reviewed-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Sven Schnelle <svens@linux.ibm.com>
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>

arch/s390/include/asm/processor.h

@@ -314,8 +314,8 @@ static inline void __load_psw(psw_t psw)
  */
 static __always_inline void __load_psw_mask(unsigned long mask)
 {
+	psw_t psw __uninitialized;
 	unsigned long addr;
-	psw_t psw;
 
 	psw.mask = mask;