lib/zebra: remove code duplication in redist_del_instance()
Change redist_check_instance() to return a pointer instead of returning 1
on success. This way this function can be reused in redist_del_instance()
instead of duplicating the same logic there.
Also, remove unnecessary call to redist_check_instance() in
zebra_redistribute_delete().
While here, remove unnecessary cast from void* in redist_add_instance().