btrfs: zoned: remove redundant initialization of to_add

to_add is being initialized to len but this is never read as to_add is
overwritten later on. Remove the redundant initialization.

Cleans up the following clang-analyzer warning:

fs/btrfs/extent-tree.c:2769:8: warning: Value stored to 'to_add' during
its initialization is never read [clang-analyzer-deadcode.DeadStores].
Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Reviewed-by: Nikolay Borisov <nborisov@suse.com>
Signed-off-by: Jiapeng Chong <jiapeng.chong@linux.alibaba.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/extent-tree.c

@@ -2766,12 +2766,11 @@ static int unpin_extent_range(struct btrfs_fs_info *fs_info,
 		spin_unlock(&cache->lock);
 		if (!readonly && return_free_space &&
 		    global_rsv->space_info == space_info) {
-			u64 to_add = len;
-
 			spin_lock(&global_rsv->lock);
 			if (!global_rsv->full) {
-				to_add = min(len, global_rsv->size -
-					     global_rsv->reserved);
+				u64 to_add = min(len, global_rsv->size -
+						      global_rsv->reserved);
+
 				global_rsv->reserved += to_add;
 				btrfs_space_info_update_bytes_may_use(fs_info,
 						space_info, to_add);