rtnl: add helper to send if skb is not null

rtnl: add helper to send if skb is not null

This is a convenience helper for routines handling conditional rtnl
events, that is code that might send a notification depending on
rtnl_has_listeners/rtnl_notify_needed.

Instead of:
   if (skb)
      rtnetlink_send(...)

Use:
      rtnetlink_maybe_send(...)

Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: Pedro Tammela <pctammela@mojatatu.com>
Link: https://lore.kernel.org/r/20231208192847.714940-4-pctammela@mojatatu.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>