userns: allow killing tasks in your own or child userns

Changelog:
	Dec  8: Fixed bug in my check_kill_permission pointed out by
	        Eric Biederman.
	Dec 13: Apply Eric's suggestion to pass target task into kill_ok_by_cred()
	        for clarity
	Dec 31: address comment by Eric Biederman:
		don't need cred/tcred in check_kill_permission.
	Jan  1: use const cred struct.
	Jan 11: Per Bastian Blank's advice, clean up kill_ok_by_cred().
	Feb 16: kill_ok_by_cred: fix bad parentheses
	Feb 23: per akpm, let compiler inline kill_ok_by_cred

Signed-off-by: Serge E. Hallyn <serge.hallyn@canonical.com>
Acked-by: "Eric W. Biederman" <ebiederm@xmission.com>
Acked-by: Daniel Lezcano <daniel.lezcano@free.fr>
Acked-by: David Howells <dhowells@redhat.com>
Cc: James Morris <jmorris@namei.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>