mbuf: add function to read packet data

mbuf: add function to read packet data

Introduce a new function to read the packet data from an mbuf chain. It
linearizes the data if required, and also ensures that the mbuf is large
enough.

This function is used in next commits that add a software parser to
retrieve the packet type.

Signed-off-by: Olivier Matz <olivier.matz@6wind.com>