flow_dissector: Add control/reporting of encapsulation

Add an input flag to flow dissector on rather dissection should stop
when encapsulation is detected (IP/IP or GRE). Also, add a key_control
flag that indicates encapsulation was encountered during the
dissection.

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