IPoIB: Create MCGs with all attributes required by RFC

RFC 4391 ("Transmission of IP over InfiniBand (IPoIB)") says:

  If the IB multicast group does not already exist, one must be
  created first with the IPoIB link MTU.  The MGID MUST use the same
  P_Key, Q_Key, SL, MTU, and HopLimit as those used in the
  broadcast-GID.  The rest of attributes SHOULD follow the values used
  in the broadcast-GID as well.

However, the current IPoIB driver is only setting the attributes
required by the InfiniBand spec to create a multicast group, so in
particular the MTU and HopLimit are not being set.  Add these
attributes when creating MCGs, and also set the Rate attribute, since
IPoIB pays attention to that attribute as well.

Signed-off-by: Roland Dreier <rolandd@cisco.com>