When in connected mode ipoib sent broadcast pings which exceeded the mtu
size for broadcast addresses.

Add an mtu attribute to the rdma_netdev structure which ipoib sets to its
mcast mtu size.

The RDMA netdev uses this value to determine if the skb length is too long
for the mtu specified and if it is, drops the packet and logs an error
about the errant packet.

Link: https://lore.kernel.org/r/20200511160655.173205.14546.stgit@awfm-01.aw.intel.comReviewed-by: Mike Marciniszyn <mike.marciniszyn@intel.com>
Reviewed-by: Dennis Dalessandro <dennis.dalessandro@intel.com>
Signed-off-by: Gary Leshner <Gary.S.Leshner@intel.com>
Signed-off-by: Kaike Wan <kaike.wan@intel.com>
Signed-off-by: Dennis Dalessandro <dennis.dalessandro@intel.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>