Btrfs: Some fixes for batching extent insert.

In insert_extents(), when ret==1 and last is not zero, it should
check if the current inserted item is the last item in this batching
inserts. If so, it should just break from loop. If not, 'cur =
insert_list->next' will make no sense because the list is empty now,
and 'op' will point to an unexpectable place.

There are also some trivial fixs in this patch including one comment
typo error and deleting two redundant lines.

Signed-off-by: Chris Mason <chris.mason@oracle.com>

fs/btrfs/ctree.c

@@ -3041,7 +3041,6 @@ int btrfs_insert_some_items(struct btrfs_trans_handle *trans,
 	struct btrfs_item *item;
 	int ret = 0;
 	int slot;
-	int slot_orig;
 	int i;
 	u32 nritems;
 	u32 total_data = 0;
@@ -3064,7 +3063,6 @@ int btrfs_insert_some_items(struct btrfs_trans_handle *trans,
 	if (ret < 0)
 		goto out;
 
-	slot_orig = path->slots[0];
 	leaf = path->nodes[0];
 
 	nritems = btrfs_header_nritems(leaf);