[S390] use helpers for rlimits

Make sure compiler won't do weird things with limits. E.g. fetching
them twice may return 2 different values after writable limits are
implemented.

I.e. either use rlimit helpers added in
3e10e716abf3c71bdb5d86b8f507f9e72236c9cd
or ACCESS_ONCE if not applicable.

Cc: Heiko Carstens <heiko.carstens@de.ibm.com>
Cc: linux390@de.ibm.com
Cc: linux-s390@vger.kernel.org
Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

arch/s390/mm/mmap.c

@@ -40,7 +40,7 @@
 
 static inline unsigned long mmap_base(void)
 {
-	unsigned long gap = current->signal->rlim[RLIMIT_STACK].rlim_cur;
+	unsigned long gap = rlimit(RLIMIT_STACK);
 
 	if (gap < MIN_GAP)
 		gap = MIN_GAP;
@@ -61,7 +61,7 @@ static inline int mmap_is_legacy(void)
 #endif
 	return sysctl_legacy_va_layout ||
 	    (current->personality & ADDR_COMPAT_LAYOUT) ||
-	    current->signal->rlim[RLIMIT_STACK].rlim_cur == RLIM_INFINITY;
+	    rlimit(RLIMIT_STACK) == RLIM_INFINITY;
 }
 
 #ifndef CONFIG_64BIT