netfilter: bridge: fix CONFIG_NF_DEFRAG_IPV4/6 related warnings/errors

br_nf_ip_fragment() is not needed when neither CONFIG_NF_DEFRAG_IPV4 nor
CONFIG_NF_DEFRAG_IPV6 is set.

struct brnf_frag_data must be available if either CONFIG_NF_DEFRAG_IPV4
or CONFIG_NF_DEFRAG_IPV6 is set.

Fixes: efb6de9b ("netfilter: bridge: forward IPv6 fragmented packets")
Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Bernhard Thaler <bernhard.thaler@wvnet.at>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>