netfilter: nf_conntrack_bridge: Fix memory leak when error

It should be added kfree_skb_list() when err is not equal to zero
in nf_br_ip_fragment().

v2: keep this aligned with IPv6.
v3: modify iter.frag_list to iter.frag.

Fixes: 3c171f49

 ("netfilter: bridge: add connection tracking system")
Signed-off-by: Yajun Deng <yajun.deng@linux.dev>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>