astraceroute: Don't call device_mtu() twice in a row

Store the result in len_rcv right away and reuse it.

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
protos: lldp: Preliminary support for LLDP dissector

NOT TESTED YET (but compile tested)

"PRIVATE" BRANCH, PLEASE DO NOT PULL. THIS COMMIT WILL GET AMENDED TO.

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>