[NEIGH]: Move netlink neighbour bits to linux/neighbour.h

Moves netlink neighbour bits to linux/neighbour.h. Also
moves bits to be exported to userspace from net/neighbour.h
to linux/neighbour.h and removes __KERNEL__ guards, userspace
is not supposed to be using it.

rtnetlink_rcv_msg() is not longer required to parse attributes
for the neighbour layer, remove dependency on obsolete and
buggy rta_buf.

Signed-off-by: Thomas Graf <tgraf@suug.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>