s390: workaround invalid gcc-11 out of bounds read warning

GCC 11.1.0 and 11.2.0 generate a wrong warning when compiling the
kernel e.g. with allmodconfig:

arch/s390/kernel/setup.c: In function ‘setup_lowcore_dat_on’:
./include/linux/fortify-string.h:57:33: error: ‘__builtin_memcpy’ reading 128 bytes from a region of size 0 [-Werror=stringop-overread]
...
arch/s390/kernel/setup.c:526:9: note: in expansion of macro ‘memcpy’
  526 |         memcpy(abs_lc->cregs_save_area, S390_lowcore.cregs_save_area,
      |         ^~~~~~

This could be addressed by using absolute_pointer() with the
S390_lowcore macro, but this is not a good idea since this generates
worse code for performance critical paths.

Therefore simply use a for loop to copy the array in question and get
rid of the warning.
Reported-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>

arch/s390/kernel/setup.c

@@ -508,6 +508,7 @@ static void __init setup_lowcore_dat_on(void)
 {
 	struct lowcore *abs_lc;
 	unsigned long flags;
+	int i;
 
 	__ctl_clear_bit(0, 28);
 	S390_lowcore.external_new_psw.mask |= PSW_MASK_DAT;
@@ -523,8 +524,8 @@ static void __init setup_lowcore_dat_on(void)
 	abs_lc = get_abs_lowcore(&flags);
 	abs_lc->restart_flags = RESTART_FLAG_CTLREGS;
 	abs_lc->program_new_psw = S390_lowcore.program_new_psw;
-	memcpy(abs_lc->cregs_save_area, S390_lowcore.cregs_save_area,
-	       sizeof(abs_lc->cregs_save_area));
+	for (i = 0; i < 16; i++)
+		abs_lc->cregs_save_area[i] = S390_lowcore.cregs_save_area[i];
 	put_abs_lowcore(abs_lc, flags);
 }