userns: add a user namespace owner of ipc ns

userns: add a user namespace owner of ipc ns

Changelog:
	Feb 15: Don't set new ipc->user_ns if we didn't create a new
		ipc_ns.
	Feb 23: Move extern declaration to ipc_namespace.h, and group
		fwd declarations at top.

Signed-off-by: Serge E. Hallyn <[email protected]>
Acked-by: "Eric W. Biederman" <[email protected]>
Acked-by: Daniel Lezcano <[email protected]>
Acked-by: David Howells <[email protected]>
Cc: James Morris <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>