x86/traps: Use SEND_SIG_PRIV instead of force_sig()

force_sig() is just force_sig_info(SEND_SIG_PRIV). Imho it should die,
we have too many ugly "send signal" helpers.

And do_trap() looks just ugly because it uses force_sig_info() or
force_sig() depending on info != NULL.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>