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.org
Signed-off-by: Paolo Abeni <pabeni@redhat.com>