s390/mm: Explicitly compare PAGE_DEFAULT_KEY against zero in storage_key_init_range

Clang warns:

 In file included from ../arch/s390/purgatory/purgatory.c:10:
 In file included from ../include/linux/kexec.h:18:
 In file included from ../include/linux/crash_core.h:6:
 In file included from ../include/linux/elfcore.h:5:
 In file included from ../include/linux/user.h:1:
 In file included from ../arch/s390/include/asm/user.h:11:
 ../arch/s390/include/asm/page.h:45:6: warning: converting the result of
 '<<' to a boolean always evaluates to false
 [-Wtautological-constant-compare]
         if (PAGE_DEFAULT_KEY)
            ^
 ../arch/s390/include/asm/page.h:23:44: note: expanded from macro
 'PAGE_DEFAULT_KEY'
 #define PAGE_DEFAULT_KEY        (PAGE_DEFAULT_ACC << 4)
                                                  ^
 1 warning generated.

Explicitly compare this against zero to silence the warning as it is
intended to be used in a boolean context.

Fixes: de3fa841 ("s390/mm: fix compile for PAGE_DEFAULT_KEY != 0")
Link: https://github.com/ClangBuiltLinux/linux/issues/860
Link: https://lkml.kernel.org/r/20200214064207.10381-1-natechancellor@gmail.comAcked-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Nathan Chancellor <natechancellor@gmail.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>

arch/s390/include/asm/page.h

@@ -42,7 +42,7 @@ void __storage_key_init_range(unsigned long start, unsigned long end);
 
 static inline void storage_key_init_range(unsigned long start, unsigned long end)
 {
-	if (PAGE_DEFAULT_KEY)
+	if (PAGE_DEFAULT_KEY != 0)
 		__storage_key_init_range(start, end);
 }