flow_dissector: Add flag to stop parsing at L3

Add an input flag to flow dissector on rather dissection should be
stopped when an L3 packet is encountered. This would be useful if a
caller just wanted to get IP addresses of the outermost header (e.g.
to do an L3 hash).

Signed-off-by: Tom Herbert <tom@herbertland.com>
Signed-off-by: David S. Miller <davem@davemloft.net>