netdevice: add missing dm_private kdoc

Building htmldocs complains:
  include/linux/netdevice.h:2295: warning: Function parameter or member 'dm_private' not described in 'net_device'

Fixes: b26ef81c ("drop_monitor: remove quadratic behavior")
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Link: https://lore.kernel.org/r/20220322051053.1883186-1-kuba@kernel.orgSigned-off-by: Paolo Abeni <pabeni@redhat.com>

include/linux/netdevice.h

@@ -1899,6 +1899,8 @@ enum netdev_ml_priv_type {
  *	@garp_port:	GARP
  *	@mrp_port:	MRP
  *
+ *	@dm_private:	Drop monitor private
+ *
  *	@dev:		Class/net/name entry
  *	@sysfs_groups:	Space for optional device, statistics and wireless
  *			sysfs groups