s390: fix system call single stepping

When single stepping an svc instruction on s390, the kernel is entered
with a PER program check interruption. The program check handler than
jumps to the system call handler by reloading the PSW. The code didn't
set GPR13 to the thread pointer in struct task_struct. This made the
kernel access invalid memory while trying to fetch the syscall function
address. Fix this by always assigned GPR13 after .Lsysc_per.

Fixes: 0b0ed657 ("s390: remove critical section cleanup from entry.S")
Reported-and-tested-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Sven Schnelle <svens@linux.ibm.com>
Reviewed-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>

arch/s390/kernel/entry.S

@@ -378,9 +378,9 @@ ENTRY(system_call)
 	stmg	%r8,%r15,__LC_SAVE_AREA_SYNC
 	BPOFF
 	lg	%r12,__LC_CURRENT
-	lghi	%r13,__TASK_thread
 	lghi	%r14,_PIF_SYSCALL
 .Lsysc_per:
+	lghi	%r13,__TASK_thread
 	lg	%r15,__LC_KERNEL_STACK
 	la	%r11,STACK_FRAME_OVERHEAD(%r15)	# pointer to pt_regs
 	UPDATE_VTIME %r8,%r9,__LC_SYNC_ENTER_TIMER