net: allow to kill a task which waits net_mutex in copy_new_ns

net_mutex can be locked for a long time. It may be because many
namespaces are being destroyed or many processes decide to create
a network namespace.

Both these operations are heavy, so it is better to have an ability to
kill a process which is waiting net_mutex.

Cc: "David S. Miller" <davem@davemloft.net>
Cc: Eric W. Biederman <ebiederm@xmission.com>
Signed-off-by: Andrei Vagin <avagin@openvz.org>
Signed-off-by: David S. Miller <davem@davemloft.net>