Merge branch 'improve-neigh_flush_dev-performance'

Merge branch 'improve-neigh_flush_dev-performance'

Gilad Naaman says:

====================
Improve neigh_flush_dev performance

This patchsets improves the performance of neigh_flush_dev.

Currently, the only way to implement it requires traversing
all neighbours known to the kernel, across all network-namespaces.

This means that some flows are slowed down as a function of neigh-scale,
even if the specific link they're handling has little to no neighbours.

In order to solve this, this patchset adds a netdev->neighbours list,
as well as making the original linked-list doubly-, so that it is
possible to unlink neighbours without traversing the hash-bucket to
obtain the previous neighbour.

The original use-case we encountered was mass-deletion of links (12K
VLANs) while there are 50K ARPs and 50K NDPs in the system; though the
slowdowns would also appear when the links are set down.
====================

Link: https://patch.msgid.link/20241107160444.2913124-1-gnaaman@drivenets.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>