IB/iser: Set IP_CSUM as default guard type

In the future this will be a per-command parameter so we can lose it,
but in the mean time IP_CSUM is a lot lighter for SW layers to
compute, set it as default.

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