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

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>
Orabug: 25395066
(cherry picked from commit 20c08b362f4b0c41103fe9d75c61ca348d021441)
Signed-off-by: Kyle Fortin <kyle.fortin@oracle.com>
Reviewed-by: Shan Hai <shan.hai@oracle.com>