IB/iser: Move PI context alloc/free to routines

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

This patch does not change any functionality.

Signed-off-by: Sagi Grimberg <sagig@mellanox.com>
Signed-off-by: Adir Lev <adirl@mellanox.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>