Monitor: don't open md array that doesn't exist.

Monitor: don't open md array that doesn't exist.

Opening a block-special-device for an array that doesn't
exist causes that array to be instantiated (as an empty array).
Races at array shutdown can cause the array to spontaneously
re-appear if some deamon notices a 'change' event and goes
to investigate.

Teach "mdadm --monitor" to avoid this race by checking the
"array_state" before opening the device.

Reported-by: Francis Moreau <francis.moro@gmail.com>
Signed-off-by: NeilBrown <neilb@suse.de>