netfilter: flowtable: use atomic bitwise operations for flow flags

Originally, all flow flag bits were set on only from the workqueue. With
the introduction of the flow teardown state and hardware offload this is
no longer true. Let's be safe and use atomic bitwise operation to
operation with flow flags.

Fixes: 59c466dd ("netfilter: nf_flow_table: add a new flow state for tearing down offloading")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>