ixgbe: check rxd number to avoid mbuf leak

ixgbe: check rxd number to avoid mbuf leak

The mbuf leak happens when the assigned number of rx descriptor is not
power of 2 in vector mode.
As it's presumed on vpmd rx (for rx_tail wrap), adding condition check
to prevent it.
The root cause reference code in *_recv_raw_pkts_vec* as below.
"rxq->rx_tail = (uint16_t)(rxq->rx_tail & (rxq->nb_rx_desc - 1));".

Reported-by: Stephen Hemminger <stephen@networkplumber.org>
Signed-off-by: Cunming Liang <cunming.liang@intel.com>
Acked-by: Bruce Richardson <bruce.richardson@intel.com>