filestore: fix rollback safety check

filestore: fix rollback safety check

There is a window in the old check between when current/commit_op_seq is
written and the snapshot is taken.  If ceph-osd crashes, we'll be unable to
start because we'll believe current/ was in use without proper checkpoints.

Instead, make the snapped/not snapped state of current/ explicit.

Fixes: #2118
Signed-off-by: Sage Weil <sage@newdream.net>
Reviewed-by: Samuel Just <samuel.just@dreamhost.com>
Reviewed-by: Yehuda Sadeh <yehuda.sadeh@dreamhost.com>