netdevsim: support taking immediate snapshot via devlink

Implement the .snapshot region operation for the dummy data region. This
enables a region snapshot to be taken upon request via the new
DEVLINK_CMD_REGION_SNAPSHOT command.

Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Reviewed-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>