vhost: fix IPv4 checksum

vhost: fix IPv4 checksum

Currently the IPv4 header checksum is calculated including its
current value, which can be a valid checksum or just garbage.
In any case, if the original value is not zero, then the result
is always wrong.

The IPv4 checksum is defined in RFC791, page 14 says:
  Header Checksum:  16 bits

  The checksum algorithm is:
  The checksum field is the 16 bit one's complement of the one's
  complement sum of all 16 bit words in the header.  For purposes of
  computing the checksum, the value of the checksum field is zero.

Thus force the csum field to always be zero.

Fixes: b08b8cfeb2ae ("vhost: fix IP checksum")
Cc: stable@dpdk.org
Signed-off-by: Flavio Leitner <fbl@sysclose.org>
Reviewed-by: Maxime Coquelin <maxime.coquelin@redhat.com>