Merge branch 'neighbour-convert-rtm_getneigh-to-rcu-and-make-pneigh-rtnl-free'

Kuniyuki Iwashima says:

====================
neighbour: Convert RTM_GETNEIGH to RCU and make pneigh RTNL-free.

This is kind of v3 of the series below [0] but without NEIGHTBL patches.

Patch 1 ~ 4 and 9 come from the series to convert RTM_GETNEIGH to RCU.

Other patches clean up pneigh_lookup() and convert the pneigh code to
RCU + private mutex so that we can easily remove RTNL from RTM_NEWNEIGH
in the later series.

[0]: https://lore.kernel.org/netdev/20250418012727.57033-1-kuniyu@amazon.com/

v2: https://lore.kernel.org/20250712203515.4099110-1-kuniyu@google.com
v1: https://lore.kernel.org/20250711191007.3591938-1-kuniyu@google.com
====================

Link: https://patch.msgid.link/20250716221221.442239-1-kuniyu@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>