Isolate the UTS namespace's domainname and hostname back

Commit 7d69a1f4 ("remove CONFIG_UTS_NS
and CONFIG_IPC_NS") by Cedric Le Goater accidentally removed the code
that prevented the uts->hostname and uts->domainname values from being
overwritten from another namespace.

In other words, setting hostname/domainname via sysfs (echo xxx >
/proc/sys/kernel/(host|domain)name) cased the new value to be set in
init UTS namespace only.

Return the isolation back.

Signed-off-by: Pavel Emelyanov <xemul@openvz.org>
Acked-by: Cedric Le Goater <clg@fr.ibm.com>
Acked-by: Serge Hallyn <serue@us.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>