lisp: Use IP addresses rather than flow on hash failure.

When calculating the source port for the UDP header, LISP primarily
uses skb_get_hash() but needs a backup in case this fails. The
current backup is a hash of the entire flow key but this includes
many fields that probably would not be considered to be part of a
flow in many situations. It assumes that all fields, including those
not used, are zeroed out which will soon not be the case.

This switches to using a hash of the IP addresses instead, which
solves both problems. These should always be present since LISP
encapsulates L3 packets.

Signed-off-by: Jesse Gross <jesse@nicira.com>
Acked-by: Thomas Graf <tgraf@suug.ch>