Currently, if two interfaces have addresses in the same connected route,
then the order of the prefix route entries is determined by the order in
which the addresses are assigned or the links brought up.

Add IFA_RT_PRIORITY to allow user to specify the metric of the prefix
route associated with an address giving interface managers better
control of the order of prefix routes.
Signed-off-by: David Ahern <dsahern@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>