iscsi-target: add int (*iscsit_validate_params)()

Add int (*iscsit_validate_params)() to
struct iscsit_transport, iscsi-target
uses this callback for validating
conn operational parameters.

cxgbit.ko needs this callback to check
and update the value of
MAXXMITDATASEGMENTLENGTH.

Signed-off-by: Varun Prakash <varun@chelsio.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>