btrfs: remove useless used space increment during space reservation

At space-info.c:__reserve_bytes(), we increment the 'used' variable, but
then we don't use the variable anymore, making the increment pointless.
The increment became useless with commit 2e294c60 ("btrfs: simplify
the logic in need_preemptive_flushing"), so just remove it.
Reviewed-by: NJosef Bacik <josef@toxicpanda.com>
Signed-off-by: NFilipe Manana <fdmanana@suse.com>
Reviewed-by: NDavid Sterba <dsterba@suse.com>
Signed-off-by: NDavid Sterba <dsterba@suse.com>

fs/btrfs/space-info.c

@@ -1686,7 +1686,6 @@ static int __reserve_bytes(struct btrfs_fs_info *fs_info,
 				      &space_info->priority_tickets);
 		}
 	} else if (!ret && space_info->flags & BTRFS_BLOCK_GROUP_METADATA) {
-		used += orig_bytes;
 		/*
 		 * We will do the space reservation dance during log replay,
 		 * which means we won't have fs_info->fs_root set, so don't do