zebra: fix redistribution of default routes

zebra: fix redistribution of default routes

We were always redistributing the default routes (IPv[46]) in
redistribute_update() because the 'client->redist_default' condition
always returns true.

The 'redist_default' member of the 'zserv' structure is a pointer and is
always initialized with vrf_bitmap_init() in the zebra_client_create()
function.

To fix this, use vrf_bitmap_check() instead.

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