IB/iser: Handle lack of memory management extentions correctly

max_fast_reg_page_list_len is only valid when the
memory management extentions are signaled by the underlying
driver.

Fix by adjusting iser_calc_scsi_params() to use
ISCSI_ISER_MAX_SG_TABLESIZE when the extentions are not indicated.

Reported-by: Thomas Rosenstein <thomas.rosenstein@creamfinance.com>
Fixes: Commit df749cdc ("IB/iser: Support up to 8MB data transfer in a single command")
Reviewed-by: Dennis Dalessandro <dennis.dalessandro@intel.com>
Signed-off-by: Mike Marciniszyn <mike.marciniszyn@intel.com>
Acked-by: Sagi Grimberg <sagi@grimberg.me>
Tested-by: Thomas Rosenstein <thomas.rosenstein@creamfinance.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>