[S390] remove reset of system call restart on psw changes

git commit 20b40a794baf3b4b "signal race with restarting system calls"
added code to the poke_user/poke_user_compat to reset the system call
restart information in the thread-info if the PSW address is changed.
The purpose of that change has been to workaround old gdbs that do
not know about the REGSET_SYSTEM_CALL. It turned out that this is not
a good idea, it makes the behaviour of the debuggee dependent on the
order of specific ptrace call, e.g. the REGSET_SYSTEM_CALL register
set needs to be written last. And the workaround does not really fix
old gdbs, inferior calls on interrupted restarting system calls do not
work either way.

Signed-off-by: Martin Schwidefsky <[e-mail]>

arch/s390/kernel/ptrace.c

···
 		     ((data & PSW_MASK_EA) && !(data & PSW_MASK_BA))))
 			/* Invalid psw mask. */
 			return -EINVAL;
-		if (addr == (addr_t) &dummy->regs.psw.addr)
-			/*
-			 * The debugger changed the instruction address,
-			 * reset system call restart, see signal.c:do_signal
-			 */
-			task_thread_info(child)->system_call = 0;
-
 		*(addr_t *)((addr_t) &task_pt_regs(child)->psw + addr) = data;
 
 	} else if (addr < (addr_t) (&dummy->regs.orig_gpr2)) {
···
 			/* Transfer 31 bit amode bit to psw mask. */
 			regs->psw.mask = (regs->psw.mask & ~PSW_MASK_BA) |
 				(__u64)(tmp & PSW32_ADDR_AMODE);
-			/*
-			 * The debugger changed the instruction address,
-			 * reset system call restart, see signal.c:do_signal
-			 */
-			task_thread_info(child)->system_call = 0;
 		} else {
 			/* gpr 0-15 */
 			*(__u32*)((addr_t) &regs->psw + addr*2 + 4) = tmp;