Add support for IFCAP_NOMAP to cxgbe(4).

Since cxgbe(4) uses sglist instead of bus_dma, this required updates
to the code that generates scatter/gather lists for packets. Also,
unmapped mbufs are always sent via DMA and never as immediate data in
the payload of a work request.

Submitted by: gallatin (earlier version)
Reviewed by: gallatin, hselasky, rrs
Discussed with: np
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D20616