dpdk: Ditch MAX_PKT_BURST macro.

dpdk: Ditch MAX_PKT_BURST macro.

The MAX_PKT_BURST and NETDEV_MAX_RX_BATCH macros had a confusing
relationship.  They basically purport to do the same thing, making it
unclear which is the source of truth.

Furthermore, while NETDEV_MAX_RX_BATCH was 256, MAX_PKT_BURST was 32,
meaning we never process a batch larger than 32 packets further adding
to the confusion.

This patch resolves the issue by removing MAX_PKT_BURST completely,
and shrinking the new NETDEV_MAX_BURST macro to only 32.  This should
have no change in the execution path except shrinking a couple of
structs and memory allocations (can't hurt).

Signed-off-by: Ethan Jackson <ethan@nicira.com>
Acked-by: Daniele Di Proietto <diproiettod@vmware.com>