mds: ensure snapclient is synced before corruption check

mds: ensure snapclient is synced before corruption check

During resolve, orphan dirfrags are purged which may result in dentry commits.
If the snapclient is not sync'd (which normally happens as the last step of
up:resolve), then the snapclient indicates the last sequence is 1 which is
always less than a dentry's legitimate "first" value.

Fixes: https://tracker.ceph.com/issues/64058
Signed-off-by: Patrick Donnelly <pdonnell@redhat.com>