Merge branch 'nexthop-nexthop-dump-fixes'

Ido Schimmel says:

====================
nexthop: Nexthop dump fixes

Patches #1 and #3 fix two problems related to nexthops and nexthop
buckets dump, respectively. Patch #2 is a preparation for the third
patch.

The pattern described in these patches of splitting the NLMSG_DONE to a
separate response is prevalent in other rtnetlink dump callbacks. I
don't know if it's because I'm missing something or if this was done
intentionally to ensure the message is delivered to user space. After
commit 0642840b

 ("af_netlink: ensure that NLMSG_DONE never fails in
dumps") this is no longer necessary and I can improve these dump
callbacks assuming this analysis is correct.

No regressions in existing tests:

 # ./fib_nexthops.sh
 [...]
 Tests passed: 230
 Tests failed:   0
====================

Link: https://lore.kernel.org/r/20230808075233.3337922-1-idosch@nvidia.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>