RDMA/cma: Replace net_device pointer with index

Provide the device interface when resolving route information to
ensure that the correct outbound device is used.  This will also
simplify processing of sin6_scope_id for IPv6 support.

Based on work from:
David Wilder <dwilder@us.ibm.com>
Jason Gunthorpe <jgunthrope@obsidianresearch.com>

Signed-off-by: Sean Hefty <sean.hefty@intel.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>