install: mkfs.f2fs: specify sector size for target image size

The total sectors that we want to format is used in different meanings from
various users. This notifies its size based on 512 bytes explicitly.

Bug: 76407663
Change-Id: I20687b40a1733d3b459a45f8b64a338c37a7bc95
Signed-off-by: Jaegeuk Kim <jaegeuk@google.com>