zebra: only clear pd_reason on shutdown/sweep

zebra: only clear pd_reason on shutdown/sweep

Only clear protodown reason on shutdown/sweep, retain protodown
state.

This is to retain traditional and expected behavior with daemons
like vrrpd setting protodown. They expet it to be set on shutdown
and retained on bring up to prevent traffic from being dropped.

We must cleanup our reason code though to prevent us from blocking
others.

Signed-off-by: Stephen Worley <sworley@nvidia.com>