MFC if_msk.c, rev 1.17 if_mskreg.h, rev 1.5 to RELENG_6.
Increase a maximum segment size of DMA to 4096. Previously it used MCLBYTES for the segment size but it used too many Tx descriptors in TSO case. While I'm here adjust maximum size of the sum of all segment lengths in a given DMA mapping to 65535, the maximum size, in bytes, of a IP packet.