Introduce and use sgid_index in CM requests in ibcore.

For RoCE, when CM requests are received for RC and UD connections,
netdevice of the incoming request is unavailable. Because of that CM
requests are always forwarded to init_net namespace.

Now that we have the GID index available, introduce SGID index in
incoming CM requests and refer to the netdevice of it.

While at it fix some incorrect uses of init_net and make sure
the rdma_create_id() function stores the VNET it is passed.

Based on linux commit:
cee104334c98dd04e9dd4d9a4fa4784f7f6aada9

MFC after: 3 days
Approved by: re (gjb)
Sponsored by: Mellanox Technologies