devlink: Add health diagnose command

Add devlink health diagnose command, in order to run a diagnose
operation over a specific reporter.

It is expected from driver's callback for diagnose command to fill it
via the buffer descriptors API. Devlink will parse it and convert it to
netlink nla API in order to pass it to the user.

Signed-off-by: Eran Ben Elisha <eranbe@mellanox.com>
Reviewed-by: Moshe Shemesh <moshe@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>