Function mbuf_free_bulk

for packets to be processed in the fast path, all packets must: * 1. share the same mempool * 2. single segment * 3. reference counter == 1 * 4. the data buffer is embedded in the mbuf * (Do not use RTE_MBUF_(IN)DIRECT, since there is a difference * between DPDK 1.8 and 2.0) */