RDMA/core: Add RDMA_NLDEV_CMD_NEWLINK/DELLINK support

Add support for new LINK messages to allow adding and deleting rdma
interfaces.  This will be used initially for soft rdma drivers which
instantiate device instances dynamically by the admin specifying a netdev
device to use.  The rdma_rxe module will be the first user of these
messages.

The design is modeled after RTNL_NEWLINK/DELLINK: rdma drivers register
with the rdma core if they provide link add/delete functions.  Each driver
registers with a unique "type" string, that is used to dispatch messages
coming from user space.  A new RDMA_NLDEV_ATTR is defined for the "type"
string.  User mode will pass 3 attributes in a NEWLINK message:
RDMA_NLDEV_ATTR_DEV_NAME for the desired rdma device name to be created,
RDMA_NLDEV_ATTR_LINK_TYPE for the "type" of link being added, and
RDMA_NLDEV_ATTR_NDEV_NAME for the net_device interface to use for this
link.  The DELLINK message will contain the RDMA_NLDEV_ATTR_DEV_INDEX of
the device to delete.

Signed-off-by: Steve Wise <swise@opengridcomputing.com>
Reviewed-by: Leon Romanovsky <leonro@mellanox.com>
Reviewed-by: Michael J. Ruhl <michael.j.ruhl@intel.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>