zebra: fix infinite loop when deleting non-default vrf

zebra: fix infinite loop when deleting non-default vrf

How to reproduce the bug:
% ip link add vrf-red type vrf table 10
% ip link set dev vrf-red up
% ip rule add oif vrf-red table 10
% ip rule add iif vrf-red table 10
% ip link add name lo1 type dummy
% ip link set dev lo1 up
% ip link set dev lo1 master vrf-red
% ip link del dev vrf-red
(zebra gets stuck in an infinite loop inside work_queue_run())

Regression introduced by commit 5a8dfcd8.

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