Factor out mbuf hashing code from LAGG driver so that other network
drivers can use it. This avoids some code duplication. Add missing
default case to all switch statements while at it. Also move the
hashing of the IPv6 flow field to layer 4 because the IPv6 flow field
is constant on a per L4 connection basis and not on a per L3 network.

Differential Revision: https://reviews.freebsd.org/D1987
Sponsored by: Mellanox Technologies
MFC after: 1 month