IB/iser: Re-introduce ib_conn

Structure that describes the RDMA relates connection objects.  Static
member of iser_conn.

This patch does not change any functionality

Signed-off-by: Sagi Grimberg <sagig@mellanox.com>
Signed-off-by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>