devlink: split out netlink code

Move out the netlink glue into a separate file.
Leave the ops in the old file because we'd have to export a ton
of functions. Going forward we should switch to split ops which
will let us to put the new ops in the netlink.c file.

Pure code move, no functional changes.

Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>
Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>