IB/core: Fix endianness annotation in rdma_is_multicast_addr()

Since ipv4_addr is a big endian 32-bit number, annotate it as such.

Fixes: commit be1d325a ("IB/core: Set RoCEv2 MGID according to spec")
Signed-off-by: Bart Van Assche <bart.vanassche@wdc.com>
Reviewed-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>