[catnip] Bad Assumption on Packet Header Size

[ppenna]

Description

When creating memory pool for holding packet headers, the memory manager is assuming a wrong size for these structures:

let header_size: usize = ETHERNET2_HEADER_SIZE + IPV4_HEADER_DEFAULT_SIZE + MAX_TCP_HEADER_SIZE;

This should be at least large enough to hold the maximum possible header for a TCP packet.