s390/facilites: use stfle_fac_list array size for MAX_FACILITY_BIT

Use the actual size of the facility list array within the lowcore
structure for the MAX_FACILITY_BIT define instead of a comment which
states what this is good for. This makes it a bit harder to break
things.
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

arch/s390/include/asm/facility.h

@@ -15,7 +15,7 @@
 #include <linux/preempt.h>
 #include <asm/lowcore.h>
 
-#define MAX_FACILITY_BIT (256*8)	/* stfle_fac_list has 256 bytes */
+#define MAX_FACILITY_BIT (sizeof(((struct lowcore *)0)->stfle_fac_list) * 8)
 
 static inline int __test_facility(unsigned long nr, void *facilities)
 {