IB/iser: Optimize completion polling

Poll in batch of 16. Since we don't want it on the stack, keep under
iser completion context (iser_comp).

Signed-off-by: Sagi Grimberg <sagig@mellanox.com>
Signed-off-by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>