tools/cephfs-mirror: Fix truncated sync from stale crawl-time file size

Drop AT_STATX_DONT_SYNC from stat calls in the crawler thread that
populate SyncEntry objects. The flag could leave a stale file size from
readdir, causing datasync to truncate files to the wrong length.

For RemoteSync, use inode attrs from ceph_readdirplus_r instead
of a redundant ceph_statxat.

Fixes: https://tracker.ceph.com/issues/77393
Signed-off-by: Kotresh HR <khiremat@redhat.com>