route: ignore RTAX_HOPLIMIT of value -1

route: ignore RTAX_HOPLIMIT of value -1

Older kernels use -1 internally as indicator to use the sysctl default,
but they still export the setting. Newer kernels use 0 to indicate that
(which is why the conversion from -1 to 0 was done here), but they also
stopped exporting the value. Since the meaning of -1 is clear, treat it
equally like default on newer kernels (which is to not print anything).

Signed-off-by: Phil Sutter <phil@nwl.cc>