[NET]: Add network namespace clone & unshare support.

This patch allows you to create a new network namespace
using sys_clone, or sys_unshare.

As the network namespace is still experimental and under development
clone and unshare support is only made available when CONFIG_NET_NS is
selected at compile time.

As this patch introduces network namespace support into code paths
that exist when the CONFIG_NET is not selected there are a few
additions made to net_namespace.h to allow a few more functions
to be used when the networking stack is not compiled in.

Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/kernel/fork.c b/kernel/fork.c
index 33f12f4..5e67f90 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -1608,7 +1608,8 @@
 	err = -EINVAL;
 	if (unshare_flags & ~(CLONE_THREAD|CLONE_FS|CLONE_NEWNS|CLONE_SIGHAND|
 				CLONE_VM|CLONE_FILES|CLONE_SYSVSEM|
-				CLONE_NEWUTS|CLONE_NEWIPC|CLONE_NEWUSER))
+				CLONE_NEWUTS|CLONE_NEWIPC|CLONE_NEWUSER|
+				CLONE_NEWNET))
 		goto bad_unshare_out;
 
 	if ((err = unshare_thread(unshare_flags)))