btrfs-progs: tune: fix the missing close() of filesystem fd

[BUG]
In "btrfs tune" subcommand, we're using open_ctree_fd(), which
requires passing a valid fd, and the caller is responsible to properly
handling the lifespan of the fd.

But we just call close_ctree() and btrfs_close_all_devices(), without
closing the fd.

[FIX]
Just manually close the fd.

Reviewed-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

tune/main.c

@@ -535,6 +535,7 @@ out:
 	}
 	close_ctree(root);
 	btrfs_close_all_devices();
+	close(fd);
 
 free_out:
 	return ret;