rib: constify arguments

The getter functions should take a constant pointer
to make it clear that node is not modified.

The rib create functions do not modify their config structure.
Mark the config as constant so that programs can pass
simple constant data.

Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>
Acked-by: Vladimir Medvedkin <vladimir.medvedkin@intel.com>