netfilter: nf_flow_table: fix missing error check for rhashtable_insert_fast

rhashtable_insert_fast() may return an error value when memory
allocation fails, but flow_offload_add() does not check for errors.
This patch just adds missing error checking.

Fixes: ac2a6666 ("netfilter: add generic flow table infrastructure")
Signed-off-by: Taehee Yoo <ap420073@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>