zebra: make rib_add() a simple wrapper for rib_add_multipath()

zebra: make rib_add() a simple wrapper for rib_add_multipath()

Both function were very similar, and as we know code duplication is not
good. As an example, in the past couple of weeks some fixes were made
on rib_add() but not on rib_add_multipath(), causing known bugs to still
exist in a different form.

Instead of merging the two functions into one, let's make rib_add()
call rib_add_multipath() with the appropriate parameters. This way we
remove the code duplication but still keep the easy-to-use rib_add()
function for single-path routes.

Signed-off-by: Renato Westphal <renato@opensourcerouting.org>