net/iavf: count continuous DD bits for Arm

net/iavf: count continuous DD bits for Arm

On Arm platforms, reading of descriptors may be re-ordered causing the
status of DD bits to be discontinuous. Add logic to only process
continuous descriptors by checking DD bits.

Fixes: 1060591eada5 ("net/avf: enable bulk allocate Rx")
Cc: stable@dpdk.org
Signed-off-by: Kathleen Capella <kathleen.capella@arm.com>
Reviewed-by: Honnappa Nagarahalli <honnappa.nagarahalli@arm.com>
Reviewed-by: Qi Zhang <qi.z.zhang@intel.com>