netfilter: nft_flow_offload: fix underflow in flowtable reference counter

The .deactivate and .activate interfaces already deal with the reference
counter. Otherwise, this results in spurious "Device is busy" errors.

Fixes: a3c90f7a ("netfilter: nf_tables: flow offload expression")
Signed-off-by: wenxu <wenxu@ucloud.cn>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>