pidns: make killed children autoreap

pidns: make killed children autoreap

Force SIGCHLD handling to SIG_IGN so that signals are not generated and so
that the children autoreap.  This increases the parallelize and in general
the speed of network namespace shutdown.

Note self reaping childrean can exist past zap_pid_ns_processess but they
will all be reaped before we allow the pid namespace init task with pid ==
1 to be reaped.

[[email protected]: checkpatch fixes]
Signed-off-by: Eric W. Biederman <[email protected]>
Cc: Oleg Nesterov <[email protected]>
Cc: Pavel Emelyanov <[email protected]>
Cc: Cyrill Gorcunov <[email protected]>
Cc: Louis Rilling <[email protected]>
Cc: Mike Galbraith <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>