Btrfs: fix the amount of space reserved for unlink

Our unlink reservations were a bit much, we were reserving 10 and I only count 8
possible items we're touching, so comment what we're reserving for and fix the
count value.  Thanks,
Signed-off-by: NJosef Bacik <josef@redhat.com>

fs/btrfs/inode.c

@@ -2758,7 +2758,16 @@ static struct btrfs_trans_handle *__unlink_start_trans(struct inode *dir,
 	u64 ino = btrfs_ino(inode);
 	u64 dir_ino = btrfs_ino(dir);
 
-	trans = btrfs_start_transaction(root, 10);
+	/*
+	 * 1 for the possible orphan item
+	 * 1 for the dir item
+	 * 1 for the dir index
+	 * 1 for the inode ref
+	 * 1 for the inode ref in the tree log
+	 * 2 for the dir entries in the log
+	 * 1 for the inode
+	 */
+	trans = btrfs_start_transaction(root, 8);
 	if (!IS_ERR(trans) || PTR_ERR(trans) != -ENOSPC)
 		return trans;