btrfs: fix not enough reserved space

btrfs_link() will insert 3 items(inode ref, dir name item and dir index item)
into the b+ tree and update 2 items(its inode, and parent's inode) in the b+
tree. So we should reserve space for these 5 items, not 3 items.
Reported-by: Tsutomu Itoh <t-itoh@jp.fujitsu.com>
Signed-off-by: Miao Xie <miaox@cn.fujitsu.com>
Signed-off-by: Chris Mason <chris.mason@oracle.com>

fs/btrfs/inode.c

@@ -4823,10 +4823,11 @@ static int btrfs_link(struct dentry *old_dentry, struct inode *dir,
 		goto fail;
 
 	/*
-	 * 1 item for inode ref
+	 * 2 items for inode and inode ref
 	 * 2 items for dir items
+	 * 1 item for parent inode
 	 */
-	trans = btrfs_start_transaction(root, 3);
+	trans = btrfs_start_transaction(root, 5);
 	if (IS_ERR(trans)) {
 		err = PTR_ERR(trans);
 		goto fail;