bridge: add a missing ntohs()

grec_nsrcs is in network order, we should convert to host horder in
br_multicast_igmp3_report()

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Acked-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>