Match interface with longest prefix in DHCPv4

- With nested prefixes reside on different interfaces of single host, current

  matching mechanism might return the interface with shorter prefix length

  if it appears later in the netlink message.

- Also make sure we do lease_set_interface only once, right after we

  traversed all IPv4 & IPv6 addresses.

Signed-off-by: Lung-Pin Chang <changlp@cs.nctu.edu.tw>