If we are mid-flush and a cap is migrated to another node, we need to
resend the cap flush message to the new MDS, and do so with the original
flush_seq to avoid leaking across a sync boundary.  Previously we didn't
redo the flush (we only flushed newly dirty data), which would cause a
later sync to hang forever.
Signed-off-by: Sage Weil <sage@newdream.net>