IB/cma: Separate port allocation to network namespaces

Keep a struct for each network namespace containing the IDRs for the RDMA
CM port spaces. The struct is created dynamically using the generic_net
mechanism.

This patch is internal infrastructure work for the following patches. In
this patch, init_net is statically used as the network namespace for
the new port-space API.

Signed-off-by: Haggai Eran <haggaie@mellanox.com>
Signed-off-by: Yotam Kenneth <yotamke@mellanox.com>
Signed-off-by: Shachar Raindel <raindel@mellanox.com>
Signed-off-by: Guy Shapiro <guysh@mellanox.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>