cxgbei: Support DDP for target I/O S/G lists with more than one entry.

A CAM target layer I/O CCB can use a S/G list of virtual address ranges
to describe its data buffer. This change adds zero-copy receive support
for such requests.

Sponsored by: Chelsio Communications
Differential Revision: https://reviews.freebsd.org/D29908

(cherry picked from commit 46bee8043ee2bd352d420cd573e0364ca45f813e)
(cherry picked from commit 8d2b4b2e7c1e0b10c4d49963753db31c4794dbc4)