net/ixgbe: fix UDP zero checksum on Arm

net/ixgbe: fix UDP zero checksum on Arm

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.

To fix it for the NEON path in Arm platform, change the bad UDP checksum to
unknown, and then let software application to recompute the checksum.

NICs: 82599(igb)
Driver: ixgbe(vector)
Architecture: arm64
$:./app/dpdk-testpmd -c 0x3 -w 0002:f9:00.0 -- -i
--port-topology=chained --enable-rx-cksum
test-pmd> set fwd rxonly
test-pmd> set verbose 1
test-pmd> start

1. UDP Test:
Package: (Ether()/IP()/UDP(checksum=0)
ol_flags: PKT_RX_L4_CKSUM_UNKNOWN PKT_RX_IP_CKSUM_GOOD

2. TCP Test:
Package: (Ether()/IP()/TCP(checksum=0)
ol_flags: PKT_RX_L4_CKSUM_BAD PKT_RX_IP_CKSUM_GOOD

Bugzilla ID: 629
Fixes: af75078fece3 ("first public release")
Cc: stable@dpdk.org
Reported-by: Paolo Valerio <pvalerio@redhat.com>
Signed-off-by: Feifei Wang <feifei.wang2@arm.com>
Reviewed-by: Ruifeng Wang <ruifeng.wang@arm.com>