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

rtnetlink_rcv_msg() is not longer required to parse attributes
for the neighbour tables 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>