Some small packet performance improvements

If the packet is smaller than MTU, disable the TSO flags.
Move TCP header parsing inside the IS_TSO?() test.
Add a new IFLIB_NEED_ZERO_CSUM flag to indicate the checksums need to be zeroed before TX.

Reviewed by: sbruno
Approved by: sbruno (mentor)
Sponsored by: Limelight Networks
Differential Revision: https://reviews.freebsd.org/D12442