IB/iser: Fix possible memory leak in iser_create_frwr_pool()

Fix leak where desc is not being freed in error flows.

Signed-off-by: Roi Dayan <roid@mellanox.com>
Signed-off-by: Sagi Grimberg <sagig@mellanox.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>