Btrfs: Free free_space item properly in btrfs_trim_block_group()

Since commit dc89e9824464e91fa0b06267864ceabe3186fd8b, we've changed
to use a specific slab for alocation of free_space items.

Signed-off-by: Li Zefan <[e-mail]>
Signed-off-by: Chris Mason <[e-mail]>

fs/btrfs/free-space-cache.c

···
 			start = entry->offset;
 			bytes = min(entry->bytes, end - start);
 			unlink_free_space(block_group, entry);
-			kfree(entry);
+			kmem_cache_free(btrfs_free_space_cachep, entry);
 		}
 
 		spin_unlock(&block_group->tree_lock);