lib/zebra: remove code duplication in redist_del_instance()

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().

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