/proc/PID/status: show all sets of pid according to ns

/proc/PID/status: show all sets of pid according to ns

If some issues occurred inside a container guest, host user could not know
which process is in trouble just by guest pid: the users of container
guest only knew the pid inside containers.  This will bring obstacle for
trouble shooting.

This patch adds four fields: NStgid, NSpid, NSpgid and NSsid:

a) In init_pid_ns, nothing changed;

b) In one pidns, will tell the pid inside containers:
  NStgid: 21776   5       1
  NSpid:  21776   5       1
  NSpgid: 21776   5       1
  NSsid:  21729   1       0
  ** Process id is 21776 in level 0, 5 in level 1, 1 in level 2.

c) If pidns is nested, it depends on which pidns are you in.
  NStgid: 5       1
  NSpid:  5       1
  NSpgid: 5       1
  NSsid:  1       0
  ** Views from level 1

[[email protected]: add CONFIG_PID_NS ifdef]
Signed-off-by: Chen Hanxiao <[email protected]>
Acked-by: Serge Hallyn <[email protected]>
Acked-by: "Eric W. Biederman" <[email protected]>
Tested-by: Serge Hallyn <[email protected]>
Tested-by: Nathan Scott <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>