Merge branch 'netlink-emsgsize'

Jakub Kicinski says:

====================
netlink: handle EMSGSIZE errors in the core

Ido discovered some time back that we usually force NLMSG_DONE
to be delivered in a separate recv() syscall, even if it would
fit into the same skb as data messages. He made nexthop try
to fit DONE with data in commit 8743aeff

 ("nexthop: Fix
infinite nexthop bucket dump when using maximum nexthop ID"),
and nobody has complained so far.

We have since also tried to follow the same pattern in new
genetlink families, but explaining to people, or even remembering
the correct handling ourselves is tedious.

Let the netlink socket layer consume -EMSGSIZE errors.
Practically speaking most families use this error code
as "dump needs more space", anyway.

v2:
 - init err to 0 in last patch
v1: https://lore.kernel.org/all/20240301012845.2951053-1-kuba@kernel.org/
====================

Signed-off-by: David S. Miller <davem@davemloft.net>