netfilter: nf_tables: fix percpu memory leak at nf_tables_addchain()

It seems to me that percpu memory for chain stats started leaking since
commit 3bc158f8

 ("netfilter: nf_tables: map basechain priority to
hardware priority") when nft_chain_offload_priority() returned an error.

Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Fixes: 3bc158f8

 ("netfilter: nf_tables: map basechain priority to hardware priority")
Signed-off-by: Florian Westphal <fw@strlen.de>