net/ixgbe: fix out of order Rx read

net/ixgbe: fix out of order Rx read

In vPMD, when we load Rx desc with _mm_loadu_si128, the volatile
modifier will be cast away, allowing the compiler to reorder the
load instructions.

The Rx recv function's correctness is relying on these load
instructions following a strict sequence, reading the descriptors
in reverse order, so we add compiler barrier to prevent compiler
reorder.

Fixes: c95584dc2b18 ("ixgbe: new vectorized functions for Rx/Tx")

Signed-off-by: Qi Zhang <qi.z.zhang@intel.com>
Acked-by: Konstantin Ananyev <konstantin.ananyev@intel.com>