neighbour: Define neigh_for_each_in_bucket

Introduce neigh_for_each_in_bucket in neighbour.h, to help iterate over
the neighbour table more succinctly.

Signed-off-by: Gilad Naaman <gnaaman@drivenets.com>
Reviewed-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20241107160444.2913124-3-gnaaman@drivenets.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>