Merge branch 'add-support-for-externally-validated-neighbor-entries'

Ido Schimmel says:

====================
Add support for externally validated neighbor entries

Patch #1 adds a new neighbor flag ("extern_valid") that prevents the
kernel from invalidating or removing a neighbor entry, while allowing
the kernel to notify user space when the entry becomes reachable. See
motivation and implementation details in the commit message.

Patch #2 adds a selftest.

v1: https://lore.kernel.org/20250611141551.462569-1-idosch@nvidia.com
====================

Link: https://patch.msgid.link/20250626073111.244534-1-idosch@nvidia.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>