Set RoCEv2 MGID according to spec in ibcore.

RoCEv2 Annex states that for RoCEv2 over IPv4, the corresponding
IPv4 address is encoded into the GID according to the following rule:
GID= :ffff:<IPv4 address>

Remove the 0xff0e prefix for RoCEv2 packets with IPv4 and leave it
zeroed and change rdma_is_multicast_addr() to consider the new logic.

Linux commit:
be1d325a335840a86c133a56c6a911c368bac0fd
1c3aea2bc8f0b2e5b57375ead40457ff75a3a2ec

MFC after: 1 week
Sponsored by: Mellanox Technologies