netfilter: nf_flow_table: simplify nf_flow_offload_gc_step()

nf_flow_offload_gc_step() and nf_flow_table_iterate() are very similar.
so that many duplicate code can be removed.
After this patch, nf_flow_offload_gc_step() is simple callback function of
nf_flow_table_iterate() like nf_flow_table_do_cleanup().

Signed-off-by: Taehee Yoo <ap420073@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>