netfilter: flowtable: add a 'default' case to flowtable datapath

Add a 'default' case in case return a uninitialized value of ret, this
should not ever happen since the follow transmit path types:

- FLOW_OFFLOAD_XMIT_UNSPEC
- FLOW_OFFLOAD_XMIT_TC

are never observed from this path. Add this check for safety reasons.

Signed-off-by: Li Qiong <liqiong@nfschina.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>