btrfs: Explicitly reserve space for devreplace item

Part of device replace involves writing an item to the device root
containing information about pending replace operations. Currently space
for this item is not being explicitly reserved so this works thanks to
presence of global reserve. While not fatal it's not a good practice.
Let's be explicit about space requirement of device replace and reserve
space when starting the transaction.
Reviewed-by: NJohannes Thumshirn <jthumshirn@suse.de>
Signed-off-by: NNikolay Borisov <nborisov@suse.com>
Reviewed-by: NDavid Sterba <dsterba@suse.com>
Signed-off-by: NDavid Sterba <dsterba@suse.com>

fs/btrfs/dev-replace.c

@@ -477,8 +477,8 @@ static int btrfs_dev_replace_start(struct btrfs_fs_info *fs_info,
 
 	btrfs_wait_ordered_roots(fs_info, U64_MAX, 0, (u64)-1);
 
-	/* force writing the updated state information to disk */
-	trans = btrfs_start_transaction(root, 0);
+	/* Commit dev_replace state and reserve 1 item for it. */
+	trans = btrfs_start_transaction(root, 1);
 	if (IS_ERR(trans)) {
 		ret = PTR_ERR(trans);
 		down_write(&dev_replace->rwsem);