Fix Tx offloads for fragmented pkt headers in ENA

If the headers of the packets are split into multiple segments of the
mbuf chain, the previous version of ena_tx_csum which was assuming,
that all segments will lay in the first mbuf, will eventually fail to
map the headers properties to meta descriptor.

That will cause Tx checksum offload to do not work and was leading to
memory corruption. It could even cause the crash of the system.

Submitted by: Michal Krawczyk <mk@semihalf.com>
Obtained from: Semihalf
Sponsored by: Amazon, Inc.