os/FileJournal: stop aio completion thread *after* writer thread

The writer thread may submit a new aio to update the header in its
final moments before shutting down.  Do not stop the aio thread until after
that has happened or else we may not wait for those aio completions.

Signed-off-by: Sage Weil <sage@redhat.com>
Revert "os/FileJournal: For journal-aio-mode, don't use aio when closing journal."

This reverts commit e870fd09ce846e5642db268c33bbe8e2e17ffef2.

The aio completion thread will now wait for the final header update aio to
finish; this workaround is no longer necessary.

Signed-off-by: Sage Weil <sage@redhat.com>