Instead of open-coding connection fastreg pool get/put,
we introduce iser_reg_desc[get|put] helpers.

We aren't setting these static as this will be a per-device
routine later on. Also, cleanup iser_unreg_rdma_mem_fastreg
a bit.

This patch does not change any functionality.
Signed-off-by: NSagi Grimberg <sagig@mellanox.com>
Signed-off-by: NAdir Lev <adirl@mellanox.com>
Signed-off-by: NDoug Ledford <dledford@redhat.com>