IB/iser: Move fast_reg_descriptor initialization to a function

fastreg descriptor will include protection information context.  In
order to place the logic in one place we introduce iser_create_fr_desc
function.

This patch does not change any functionality.

Signed-off-by: Sagi Grimberg <sagig@mellanox.com>
Signed-off-by: Alex Tabachnik <alext@mellanox.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>