Btrfs: ensure tmpfile inode is always persisted with link count of 0

If we open a file with O_TMPFILE, don't do any further operation on
it (so that the inode item isn't updated) and then force a transaction
commit, we get a persisted inode item with a link count of 1, and not 0
as it should be.

Steps to reproduce it (requires a modern xfs_io with -T support):

    $ mkfs.btrfs -f /dev/sdd
    $ mount -o /dev/sdd /mnt
    $ xfs_io -T /mnt &
    $ sync

Then btrfs-debug-tree shows the inode item with a link count of 1:

    $ btrfs-debug-tree /dev/sdd
    (...)
    fs tree key (FS_TREE ROOT_ITEM 0)
    leaf 29556736 items 4 free space 15851 generation 6 owner 5
    fs uuid f164d01b-1b92-481d-a4e4-435fb0f843d0
    chunk uuid 0e3d0e56-bcca-4a1c-aa5f-cec2c6f4f7a6
    	item 0 key (256 INODE_ITEM 0) itemoff 16123 itemsize 160
		inode generation 3 transid 6 size 0 block group 0 mode 40755 links 1
    	item 1 key (256 INODE_REF 256) itemoff 16111 itemsize 12
    		inode ref index 0 namelen 2 name: ..
    	item 2 key (257 INODE_ITEM 0) itemoff 15951 itemsize 160
    		inode generation 6 transid 6 size 0 block group 0 mode 100600 links 1
    	item 3 key (ORPHAN ORPHAN_ITEM 257) itemoff 15951 itemsize 0
		orphan item
    checksum tree key (CSUM_TREE ROOT_ITEM 0)
    (...)
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Chris Mason <clm@fb.com>

fs/btrfs/inode.c

@@ -5641,6 +5641,13 @@ static struct inode *btrfs_new_inode(struct btrfs_trans_handle *trans,
 		return ERR_PTR(-ENOMEM);
 	}
 
+	/*
+	 * O_TMPFILE, set link count to 0, so that after this point,
+	 * we fill in an inode item with the correct link count.
+	 */
+	if (!name)
+		set_nlink(inode, 0);
+
 	/*
 	 * we have to initialize this early, so we can reclaim the inode
 	 * number if we fail afterwards in this function.
@@ -9007,6 +9014,14 @@ static int btrfs_tmpfile(struct inode *dir, struct dentry *dentry, umode_t mode)
 	if (ret)
 		goto out;
 
+	/*
+	 * We set number of links to 0 in btrfs_new_inode(), and here we set
+	 * it to 1 because d_tmpfile() will issue a warning if the count is 0,
+	 * through:
+	 *
+	 *    d_tmpfile() -> inode_dec_link_count() -> drop_nlink()
+	 */
+	set_nlink(inode, 1);
 	d_tmpfile(dentry, inode);
 	mark_inode_dirty(inode);