s390: avoid undefined behaviour

At a couple of places smatch emits warnings like this:

    arch/s390/mm/vmem.c:409 vmem_map_init() warn:
        right shifting more than type allows

In fact shifting a signed type right is undefined. Avoid this and add
an unsigned long cast. The shifted values are always positive.

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>

arch/s390/mm/init.c

@@ -144,8 +144,8 @@ void __init mem_init(void)
 
 void free_initmem(void)
 {
-	__set_memory((unsigned long) _sinittext,
-		     (_einittext - _sinittext) >> PAGE_SHIFT,
+	__set_memory((unsigned long)_sinittext,
+		     (unsigned long)(_einittext - _sinittext) >> PAGE_SHIFT,
 		     SET_MEMORY_RW | SET_MEMORY_NX);
 	free_initmem_default(POISON_FREE_INITMEM);
 }