md: test mddev->flags more safely in md_check_recovery.

commit 7a0a5355 md: Don't test all of mddev->flags at once.
made most tests on mddev->flags safer, but missed one.

When
commit 260fa034 md: avoid deadlock when dirty buffers during md_stop.
added MD_STILL_CLOSED, this caused md_check_recovery to misbehave.
It can think there is something to do but find nothing.  This can
lead to the md thread spinning during array shutdown.

https://bugzilla.kernel.org/show_bug.cgi?id=65721



Reported-and-tested-by: Richard W.M. Jones <rjones@redhat.com>
Fixes: 260fa034
Cc: stable@vger.kernel.org (3.12)
Signed-off-by: NeilBrown <neilb@suse.de>