zebra: create an abstraction layer for netlink_neigh_update()

zebra: create an abstraction layer for netlink_neigh_update()

We shouldn't have platform-agnostic code (e.g. zebra/interface.c)
calling platform-specific functions (e.g. netlink_neigh_update).

This commit introduces the kernel_neigh_update() function, which then
has to be implemented by all supported platforms. Currently only Linux
implements this function, which is only used by the RTADV code.

Signed-off-by: Renato Westphal <renato@opensourcerouting.org>