net: make for_each_netdev_dump() a little more bug-proof

commit f22b4b55 upstream.

I find the behavior of xa_for_each_start() slightly counter-intuitive.
It doesn't end the iteration by making the index point after the last
element. IOW calling xa_for_each_start() again after it "finished"
will run the body of the loop for the last valid element, instead
of doing nothing.

This works fine for netlink dumps if they terminate correctly
(i.e. coalesce or carefully handle NLM_DONE), but as we keep getting
reminded legacy dumps are unlikely to go away.

Fixing this generically at the xa_for_each_start() level seems hard -
there is no index reserved for "end of iteration".
ifindexes are 31b wide, tho, and iterator is ulong so for
for_each_netdev_dump() it's safe to go to the next element.

Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Reviewed-by: Przemek Kitszel <przemyslaw.kitszel@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
[ The mctp RTM_GETADDR rework backport of acab78ae ("net: mctp: Don't
  access ifa_index when missing") pulled 2d45eeb7 ("mctp: no longer
  rely on net->dev_index_head[]") as a dependency. However, that change
  relies on this backport for correct behaviour of
  for_each_netdev_dump().

  Jakub mentions[1] that nothing should be relying on the old behaviour
  of for_each_netdev_dump(), hence the backport.

  [1]: https://lore.kernel.org/netdev/20250609083749.741c27f5@kernel.org/

 ]
Signed-off-by: Jeremy Kerr <jk@codeconstruct.com.au>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>