ixgbe: fix mbuf failure statistics in vector Rx

ixgbe: fix mbuf failure statistics in vector Rx

The statistics that is reported through the rx_nombuf fields in struct
rte_eth_stats was not set when the vector PMD was used. The statistics
should report the number of mbufs that could _not_ be allocated during
rearm of the RX queue. The non-vector PMD reports it correctly. The
use of either vector PMD or non-vector PMD depends on runtime
configuration. Hence it is possible that a change in configuration
would disable this statistics. To prevent this from happening, the
statistics should be reported by both implementations.

Signed-off-by: Balazs Nemeth <balazs.nemeth@intel.com>
Acked-by: Bruce Richardson <bruce.richardson@intel.com>