s390/facility: Disable compile time optimization for decompressor code

Disable compile time optimizations of test_facility() for the
decompressor. The decompressor should not contain any optimized code
depending on the architecture level set the kernel image is compiled
for to avoid unexpected operation exceptions.

Add a __DECOMPRESSOR check to test_facility() to enforce that
facilities are always checked during runtime for the decompressor.
Reviewed-by: Sven Schnelle <svens@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>

arch/s390/include/asm/facility.h

@@ -59,8 +59,10 @@ static inline int test_facility(unsigned long nr)
 	unsigned long facilities_als[] = { FACILITIES_ALS };
 
 	if (__builtin_constant_p(nr) && nr < sizeof(facilities_als) * 8) {
-		if (__test_facility(nr, &facilities_als))
-			return 1;
+		if (__test_facility(nr, &facilities_als)) {
+			if (!__is_defined(__DECOMPRESSOR))
+				return 1;
+		}
 	}
 	return __test_facility(nr, &stfle_fac_list);
 }