btrfs: fix rw_devices miss match after seed replace

reproducer:
    reproducer:
    mount /dev/sdb /btrfs
    btrfs dev add /dev/sdc /btrfs
    btrfs rep start -B /dev/sdb /dev/sdd /btrfs
    umount /btrfs

WARNING: CPU: 0 PID: 3882 at fs/btrfs/volumes.c:892 __btrfs_close_devices+0x1c8/0x200 [btrfs]()

which is

        WARN_ON(fs_devices->rw_devices);

   The problem here is that we did not add one to the rw_devices when
   we replace the seed device with a writable device.
Signed-off-by: NAnand Jain <anand.jain@oracle.com>
Signed-off-by: NChris Mason <clm@fb.com>

fs/btrfs/dev-replace.c

@@ -562,6 +562,8 @@ static int btrfs_dev_replace_finishing(struct btrfs_fs_info *fs_info,
 	if (fs_info->fs_devices->latest_bdev == src_device->bdev)
 		fs_info->fs_devices->latest_bdev = tgt_device->bdev;
 	list_add(&tgt_device->dev_alloc_list, &fs_info->fs_devices->alloc_list);
+	if (src_device->fs_devices->seeding)
+		fs_info->fs_devices->rw_devices++;
 
 	/* replace the sysfs entry */
 	btrfs_kobj_rm_device(fs_info, src_device);