This is a further question after this one.
I google for it, but most of the search results are about how to grow the partition and the file system at the same time.
I think this question is simpler than that, because I the partition is already larger than the file system, but I want to grow the file system to fill the partition.
The reason why my file system is smaller than the partition is that I didn't format the partition by doing
mkfs on it, but rather
cating a file system image into it. And the image is smaller than the target partition.
The reason why I have to grow the file system to fully fill the partition is that my embedded SoC won't boot when there's a gap, explained at the end of this wiki.