Investigation of iSCSI 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.