zebra: Change the mechanism for comparing route ID's.

zebra: Change the mechanism for comparing route ID's.

The current format uses subtraction of two ints. Unfortunately, the
subtraction method does not work for all combinations of numbers.
For example, the with numbers represented by 10.x.x.x and 192.x.x.x,
10.x.x.x - 192.x.x.x will yield a very large positive number indicating
that 10.x.x.x is larger.

Signed-off-by: Ken Williams <kenneth.j.williams@intel.com>
Acked-by: Feng Lu <lu.feng@6wind.com>
Signed-off-by: David Lamparter <equinox@opensourcerouting.org>