signal/arm64: Use force_sig not force_sig_fault for SIGKILL

I don't think this is userspace visible but SIGKILL does not have
any si_codes that use the fault member of the siginfo union.  Correct
this the simple way and call force_sig instead of force_sig_fault when
the signal is SIGKILL.

The two know places where synchronous SIGKILL are generated are
do_bad_area and fpsimd_save.  The call paths to force_sig_fault are:
do_bad_area
  arm64_force_sig_fault
    force_sig_fault
force_signal_inject
  arm64_notify_die
    arm64_force_sig_fault
       force_sig_fault

Which means correcting this in arm64_force_sig_fault is enough
to ensure the arm64 code is not misusing the generic code, which
could lead to maintenance problems later.

Cc: stable@vger.kernel.org
Cc: Dave Martin <Dave.Martin@arm.com>
Cc: James Morse <james.morse@arm.com>
Cc: Will Deacon <will.deacon@arm.com>
Acked-by: Will Deacon <will.deacon@arm.com>
Fixes: af40ff68 ("arm64: signal: Ensure si_code is valid for all fault signals")
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>

arch/arm64/kernel/traps.c

@@ -256,6 +256,9 @@ void arm64_force_sig_fault(int signo, int code, void __user *addr,
 			   const char *str)
 {
 	arm64_show_signal(signo, str);
+	if (signo == SIGKILL)
+		force_sig(SIGKILL, current);
+	else
 		force_sig_fault(signo, code, addr, current);
 }