uclient: reset flushing_caps on (mds) cap import.

uclient: reset flushing_caps on (mds) cap import.

Previously, we could get stuck thinking that we'd flushed caps
(that went to the original MDS, waited on freeze for export,
and then were dropped) without ever telling the auth MDS that we
wanted to do so. This caused hung shutdowns:
1) during shutdown we drop all our caps
2) we get stuck and notice that we have a flushing cap
3) we send cap flush
4) MDS ignores it (I think because actual data already got updated?
and now we don't have the proper caps either)

Signed-off-by: Greg Farnum <gregory.farnum@dreamhost.com>