datapath: Fix tracking of flags seen in TCP flows.

Flow statistics need to take into account the TCP flags from the packet
currently being processed (in 'key'), not the TCP flags matched by the
flow found in the kernel flow table (in 'flow').

This bug made the Open vSwitch userspace fin_timeout action have no effect
in many cases.

Bug #1219516.
Reported-by: Len Gao <leng@vmware.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Jarno Rajahalme <jrajahalme@nicira.com>
Acked-by: Jesse Gross <jesse@nicira.com>