flow_dissector: Move skb related functions to skbuff.h

Move the flow dissector functions that are specific to skbuffs into
skbuff.h out of flow_dissector.h. This makes flow_dissector.h have
no dependencies on skbuff.h.

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