[NET]: Move netlink interface bits to linux/if_link.h.

Moving netlink interface bits to linux/if.h is rather troublesome for
applications including both linux/if.h (which was changed to be included
from linux/rtnetlink.h automatically) and net/if.h.

Signed-off-by: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>
Signed-off-by: David S. Miller <davem@davemloft.net>