signal/x86/setup_frame: __put_user cleanup

Remove the remaining check for __put_user return
value, and all the checks for err variable which
isn't set anywhere anymore.

No we can only end up in give_sigsegv due to failed
lock_user_struct - thus we remove the unlock_user_struct
to avoid unlocking a region never locked.

Signed-off-by: Riku Voipio <riku.voipio@linaro.org>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
