Btrfs: check total number of devices when removing missing

Btrfs: check total number of devices when removing missing

commit 035fe03a7ad56982b30ab3a522b7b08d58feccd0 upstream.

If you have a disk failure in RAID1 and then add a new disk to the
array, and then try to remove the missing volume, it will fail.  The
reason is the sanity check only looks at the total number of rw devices,
which is just 2 because we have 2 good disks and 1 bad one.  Instead
check the total number of devices in the array to make sure we can
actually remove the device.  Tested this with a failed disk setup and
with this test we can now run

btrfs-vol -r missing /mount/point

and it works fine.

Signed-off-by: Josef Bacik <josef@redhat.com>
Signed-off-by: Chris Mason <chris.mason@oracle.com>
Acked-by: Jeff Mahoney <jeffm@suse.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>