Set interface with longest prefix in DHCP & DHCPv6 lease

- With nested prefixes reside on different interfaces of single host

  (e.g., in 6to4, 2002::/16 on WAN and 2002:<IPv4>:<subnet>::/64 on LAN),

  current matching mechanism might return the interface with shorter prefix

  length instead of the longer one, if it appears later in the netlink message.

Signed-off-by: Lung-Pin Chang <changlp@cs.nctu.edu.tw>
Fix logic for associating leases with interfaces.

This handles the case that more than one interface contains
the network the lease address is on, but the interfaces have different
prefix lengths. Use the longest prefix length.
Tidy up previous commit.