signal: fill in si_code in prepare_kill_siginfo()

So that do_tkill() can use this helper too. This also simplifies
the next patch.

TODO: perhaps we can kill prepare_kill_siginfo() and change the
callers to use SEND_SIG_NOINFO,  but this needs some changes in
__send_signal_locked() and TP_STORE_SIGINFO().

Reviewed-by: Tycho Andersen <tandersen@netflix.com>
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Link: https://lore.kernel.org/r/20240209130620.GA8039@redhat.com


Signed-off-by: Christian Brauner <brauner@kernel.org>