net/smc: Introduce generic netlink interface for diagnostic purposes

Introduce generic netlink interface infrastructure to expose
the diagnostic information regarding smc linkgroups, links and devices.

Signed-off-by: Guvenc Gulce <guvenc@linux.ibm.com>
Signed-off-by: Karsten Graul <kgraul@linux.ibm.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>