signals: send_signal: use si_fromuser() to detect from_ancestor_ns

Change send_signal() to use si_fromuser().  From now SEND_SIG_NOINFO
triggers the "from_ancestor_ns" check.

This fixes reparent_thread()->group_send_sig_info(pdeath_signal)
behaviour, before this patch send_signal() does not detect the
cross-namespace case when the child of the dying parent belongs to the
sub-namespace.

This patch can affect the behaviour of send_sig(), kill_pgrp() and
kill_pid() when the caller sends the signal to the sub-namespace with
"priv == 0" but surprisingly all callers seem to use them correctly,
including disassociate_ctty(on_exit).

Except: drivers/staging/comedi/drivers/addi-data/*.c incorrectly use
send_sig(priv => 0).  But his is minor and should be fixed anyway.

Reported-by: Daniel Lezcano <dlezcano@fr.ibm.com>
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Cc: Roland McGrath <roland@redhat.com>
Reviewed-by: Sukadev Bhattiprolu <sukadev@us.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>