target/user: Make sure netlink would reach all network namespaces

The current code only allow netlink to reach the initial network namespace,
which caused trouble for any client running inside container.

This patch would make sure TCMU netlink would work for all network
namespaces.

Signed-off-by: Sheng Yang <sheng@yasker.org>
Acked-by: Andy Grover <agrover@redhat.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>