Isolate the UTS namespace's domainname and hostname back

Commit 7d69a1f4a72b18876c99c697692b78339d491568 ("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 <[e-mail]>
Acked-by: Cedric Le Goater <[e-mail]>
Acked-by: Serge Hallyn <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

kernel/utsname_sysctl.c

···
 static void *get_uts(ctl_table *table, int write)
 {
 	char *which = table->data;
+	struct uts_namespace *uts_ns;
+
+	uts_ns = current->nsproxy->uts_ns;
+	which = (which - (char *)&init_uts_ns) + (char *)uts_ns;
 
 	if (!write)
 		down_read(&uts_sem);