Add support for RoCEv2 multicast in ibcore.

When creating address handle from multicast GID, set MAC according to
the appropriate formula instead of searching for it in the GID table:

• For IPv4 multicast GID use ip_eth_mc_map().
• For IPv6 multicast GID use ipv6_eth_mc_map().

Linux commit:
9636a56fa864464896bf7d1272c701f2b9a57737

MFC after: 1 week
Sponsored by: Mellanox Technologies