btrfs: User assert to document transaction requirement

Using an ASSERT in btrfs_pin_extent allows to more stringently observe
whether the function is called under a transaction or not.

Reviewed-by: Johannes Thumshirn <jthumshirn@suse.de>
Signed-off-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/extent-tree.c

@@ -2590,14 +2590,13 @@ static int pin_down_extent(struct btrfs_block_group_cache *cache,
 	return 0;
 }
 
-/*
- * this function must be called within transaction
- */
 int btrfs_pin_extent(struct btrfs_fs_info *fs_info,
 		     u64 bytenr, u64 num_bytes, int reserved)
 {
 	struct btrfs_block_group_cache *cache;
 
+	ASSERT(fs_info->running_transaction);
+
 	cache = btrfs_lookup_block_group(fs_info, bytenr);
 	BUG_ON(!cache); /* Logic error */