userns: allow sethostname in a container

Changelog:
	Feb 23: let clone_uts_ns() handle setting uts->user_ns
		To do so we need to pass in the task_struct who'll
		get the utsname, so we can get its user_ns.
	Feb 23: As per Oleg's coment, just pass in tsk, instead of two
		of its members.

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>