net: bridge: use mld2r_ngrec instead of icmpv6_dataun

br_ip6_multicast_mld2_report function uses icmp6h
to parse mld2_report packet.

mld2r_ngrec defines mld2r_hdr.icmp6_dataun.un_data16[1]
in include/net/mld.h.

So, it is more compact to use mld2r rather than icmp6h.

By doing printk test, it is confirmed that
icmp6h->icmp6_dataun.un_data16[1] and mld2r->mld2r_ngrec are
indeed equivalent.

Also, sizeof(*mld2r) and sizeof(*icmp6h) are equivalent, too.

Signed-off-by: MichelleJin <shjy180909@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>