net/i40e: fix risk in descriptor read in NEON Rx

net/i40e: fix risk in descriptor read in NEON Rx

Rx descriptor is 16B/32B in size. If the DD bit is set, it indicates
that the rest of the descriptor words have valid values. Hence, the
word containing DD bit must be read first before reading the rest of
the descriptor words.

In NEON vector PMD, vector load loads two contiguous 8B of
descriptor data into vector register. Given vector load ensures no
16B atomicity, read of the word that includes DD field could be
reordered after read of other words. In this case, some words could
contain invalid data.

Read barrier is added after read of qword1 that includes DD field.
And qword0 is reloaded to update vector register. This ensures
that the fetched data is correct.

Testpmd single core test on N1SDP/ThunderX2 showed no performance drop.

Fixes: ae0eb310f253 ("net/i40e: implement vector PMD for ARM")
Cc: stable@dpdk.org
Signed-off-by: Ruifeng Wang <ruifeng.wang@arm.com>
Reviewed-by: Honnappa Nagarahalli <honnappa.nagarahalli@arm.com>