cxgb(4): Remove assumption of physically contiguous mbufs.

Investigation of iSCSI target data corruption reports brought me to
discovery that cxgb(4) expects mbufs to be physically contiguous, that
is not true after I've started using m_extaddref() in software iSCSI
for large zero-copy transmissions. In case of fragmented memory the
driver transmitted garbage from pages following the first one due to
simple use of pmap_kextract() for the first pointer instead of proper
bus_dmamap_load_mbuf_sg(). Seems like it was done as some optimization
many years ago, and at very least it is wrong in a world of IOMMUs.

This patch just removes that optimization, plus limits packet coalescing
for mbufs crossing page boundary, also depending on assumption of one
segment per packet.

MFC after: 3 days
Sponsored by: iXsystems, Inc.
Reviewed by: mmacy, np
Differential revision: https://reviews.freebsd.org/D28428