net/ixgbe: allow bulk alloc for the max size desc ring

net/ixgbe: allow bulk alloc for the max size desc ring

The only reason why bulk alloc disabled for the rings with
more than (IXGBE_MAX_RING_DESC - RTE_PMD_IXGBE_RX_MAX_BURST)
descriptors is the possible out-of-bound access to the dma
memory. But it's the artificial limit and can be easily
avoided by allocating of RTE_PMD_IXGBE_RX_MAX_BURST more
descriptors in memory. This will not interfere the HW and,
as soon as all rings' memory zeroized, Rx functions will
work correctly.

This change allows to use vectorized Rx functions with
4096 descriptors in Rx ring which is important to achieve
zero packet drop rate in high-load installations.

Signed-off-by: Ilya Maximets <i.maximets@samsung.com>
Acked-by: Jingjing Wu <jingjing.wu@intel.com>