Make iser_[create|destroy]_fastreg_desc shorter, more
readable and easily extendable.

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>