split TSO fragments larger than 16K into multiple descs

enic WQ desc supports a maximum 16K buf size, so split any send fragments
larger than 16K into several descs.

Signed-off-by: Scott Feldman <scofeldm@cisco.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
^b¤‹™/x