s390/smp: sort out physical vs virtual CPU0 lowcore pointer

SPX instruction called from set_prefix() expects physical
address of the lowcore to be installed, but instead the
virtual address is passed.

Note: this does not fix a bug currently, since virtual and
physical addresses are identical.
Reviewed-by: NHeiko Carstens <hca@linux.ibm.com>
Signed-off-by: NAlexander Gordeev <agordeev@linux.ibm.com>
Signed-off-by: NHeiko Carstens <hca@linux.ibm.com>

arch/s390/kernel/setup.c

@@ -494,7 +494,7 @@ static void __init setup_lowcore_dat_off(void)
 	lc->return_mcck_lpswe = gen_lpswe(__LC_RETURN_MCCK_PSW);
 	lc->preempt_count = PREEMPT_DISABLED;
 
-	set_prefix((u32)(unsigned long) lc);
+	set_prefix(__pa(lc));
 	lowcore_ptr[0] = lc;
 }