vtep: L3 metadata

[jerome-jutteau]

Some bricks needs to get information on L3.
Currently, metadata is only set in pollable bricks for L2 but no information is provided about L3 (it may be filled by dpdk'pmd however).
Bricks like vtep needs information about it's outer IP length. This length should be set correctly in metadata information to access L4 data as ip header may be length variable.
See src/utils/network.h