btrfs: fix EIO misuse to report missing degraded option

EIO is only for the IO failure to the device, avoid it. Use ENOENT as
that's the closest error code describing what happened.

Signed-off-by: Anand Jain <anand.jain@oracle.com>
Reviewed-by: David Sterba <dsterba@suse.com>
[ update changelog ]
Signed-off-by: David Sterba <dsterba@suse.com>