[S390] Get rid of memcpy gcc warning workaround.

Compile smp.o with -Wno-nonnull so gcc stops warning about memcpy
being used with a null parameter. Also remove the workaround code
and use a char * cast instead of a void * cast to do computations.

Cc: Bastian Blank <bastian@waldi.eu.org>
Signed-off-by: NHeiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: NMartin Schwidefsky <schwidefsky@de.ibm.com>

arch/s390/kernel/Makefile

@@ -4,6 +4,11 @@
 
 EXTRA_AFLAGS	:= -traditional
 
+#
+# Passing null pointers is ok for smp code, since we access the lowcore here.
+#
+CFLAGS_smp.o	:= -Wno-nonnull
+
 obj-y	:=  bitmap.o traps.o time.o process.o base.o early.o \
             setup.o sys_s390.o ptrace.o signal.o cpcmd.o ebcdic.o \
 	    semaphore.o s390_ext.o debug.o irq.o ipl.o dis.o diag.o

arch/s390/kernel/smp.c

@@ -629,14 +629,8 @@ static int __cpuinit smp_alloc_lowcore(int cpu)
 	panic_stack = __get_free_page(GFP_KERNEL);
 	if (!panic_stack || !async_stack)
 		goto out;
-	/*
-	 * Only need to copy the first 512 bytes from address 0. But since
-	 * the compiler emits a warning if src == NULL for memcpy use copy_page
-	 * instead. Copies more than needed but this code is not performance
-	 * critical.
-	 */
-	copy_page(lowcore, &S390_lowcore);
-	memset((void *)lowcore + 512, 0, sizeof(*lowcore) - 512);
+	memcpy(lowcore, &S390_lowcore, 512);
+	memset((char *)lowcore + 512, 0, sizeof(*lowcore) - 512);
 	lowcore->async_stack = async_stack + ASYNC_SIZE;
 	lowcore->panic_stack = panic_stack + PAGE_SIZE;