Btrfs: fix wrong max device number for single profile

The max device number of single profile is 1, not 0 (0 means 'as many as
possible'). Fix it.

Cc: Liu Bo <bo.li.liu@oracle.com>
Signed-off-by: NMiao Xie <miaox@cn.fujitsu.com>
Reviewed-by: NLiu Bo <bo.li.liu@oracle.com>
Signed-off-by: NJosef Bacik <jbacik@fusionio.com>

fs/btrfs/volumes.c

@@ -3507,7 +3507,7 @@ struct btrfs_raid_attr btrfs_raid_array[BTRFS_NR_RAID_TYPES] = {
 	{ 1, 1, 2, 2, 2, 2 /* raid1 */ },
 	{ 1, 2, 1, 1, 1, 2 /* dup */ },
 	{ 1, 1, 0, 2, 1, 1 /* raid0 */ },
-	{ 1, 1, 0, 1, 1, 1 /* single */ },
+	{ 1, 1, 1, 1, 1, 1 /* single */ },
 };
 
 static int __btrfs_alloc_chunk(struct btrfs_trans_handle *trans,