s390: fix info leak in do_sigsegv

Aleksa has reported incorrect si_errno value when stracing task which
received SIGSEGV:
[pid 20799] --- SIGSEGV {si_signo=SIGSEGV, si_code=SEGV_MAPERR, si_errno=2510266, si_addr=0x100000000000000}

The reason seems to be that do_sigsegv is not initializing siginfo
structure defined on the stack completely so it will leak 4B of
the previous stack content. Fix it simply by initializing si_errno
to 0 (same as do_sigbus does already).

Cc: stable # introduced pre-git times
Reported-by: NAleksa Sarai <asarai@suse.de>
Signed-off-by: NMichal Hocko <mhocko@suse.com>
Signed-off-by: NMartin Schwidefsky <schwidefsky@de.ibm.com>

arch/s390/mm/fault.c

@@ -250,6 +250,7 @@ static noinline void do_sigsegv(struct pt_regs *regs, int si_code)
 
 	report_user_fault(regs, SIGSEGV, 1);
 	si.si_signo = SIGSEGV;
+	si.si_errno = 0;
 	si.si_code = si_code;
 	si.si_addr = (void __user *)(regs->int_parm_long & __FAIL_ADDR_MASK);
 	force_sig_info(SIGSEGV, &si, current);