Adopting rte_pktmbuf_read()

We have written Gatekeeper without using `rte_pktmbuf_read()`, so the code assumes that the headers that need to be inspected are in the first segment of the `struct rte_mbuf` and the code explicitly checks that there are still bytes to be read in the packet whenever a new header must be inspected. Our code can drop that assumption and simplify the buffer size verification adopting `rte_pktmbuf_read()`.