cxgbei: Restructure how PDU limits are managed.

• Compute data segment limits in read_pdu_limits() rather than PDU length limits.

• Add back connection-specific PDU overhead lengths to compute PDU length limits in icl_cxgbei_conn_handoff().

Reviewed by: np
Sponsored by: Chelsio Communications
Differential Revision: https://reviews.freebsd.org/D31574