net/ixgbe: fix UDP zero checksum on x86

net/ixgbe: fix UDP zero checksum on x86

There is an 82599 errata that UDP frames with a zero checksum are
incorrectly marked as checksum invalid by the hardware.  This was
leading to misleading PKT_RX_L4_CKSUM_BAD flag.

This patch changes the bad UDP checksum to PKT_RX_L4_CKSUM_UNKNOWN,
so the software application will then have to recompute the checksum
itself if needed.

Bugzilla ID: 629
Fixes: af75078fece3 ("first public release")
Cc: stable@dpdk.org
Reported-by: Paolo Valerio <pvalerio@redhat.com>
Signed-off-by: Haiyue Wang <haiyue.wang@intel.com>
Acked-by: Konstantin Ananyev <konstantin.ananyev@intel.com>
Tested-by: Paolo Valerio <pvalerio@redhat.com>