MFC r287775:
Update TSO limits to include all headers.

To make driver programming easier the TSO limits are changed to
reflect the values used in the BUSDMA tag a network adapter driver is
using. The TCP/IP network stack will subtract space for all linklevel
and protocol level headers and ensure that the full mbuf chain passed
to the network adapter fits within the given limits. See r287775
for a more detailed description.

Differential Revision: https://reviews.freebsd.org/D3477
Reviewed by: rmacklem