IB/iser: Limit the max size of a scsi command

Currently, the data length of a command coming down from scsi-ml
is limited only by the size of its sg list (sg_tablesize). The
max data length may be different for different page size values.
By setting max_sectors, we limit the data length to
max_sectors*512 bytes.

Signed-off-by: Erez Zilber <erezz@voltaire.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>