Fix leaf overflow check in btrfs_insert_empty_items

It was incorrectly adding an extra sizeof(struct btrfs_item) and causing
false positives (oops)

Signed-off-by: Chris Mason <chris.mason@oracle.com>
diff --git a/fs/btrfs/ctree.c b/fs/btrfs/ctree.c
index ab07557..18e8447 100644
--- a/fs/btrfs/ctree.c
+++ b/fs/btrfs/ctree.c
@@ -2737,8 +2737,7 @@
 	nritems = btrfs_header_nritems(leaf);
 	data_end = leaf_data_end(root, leaf);
 
-	if (btrfs_leaf_free_space(root, leaf) <
-	    sizeof(struct btrfs_item) + total_size) {
+	if (btrfs_leaf_free_space(root, leaf) < total_size) {
 		btrfs_print_leaf(root, leaf);
 		printk("not enough freespace need %u have %d\n",
 		       total_size, btrfs_leaf_free_space(root, leaf));