make_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 4096 bytes explicitly.

Bug: 76407663
Change-Id: I4e00f2e2289c1381f0238d2a4acb606a0ab551a9
Merged-In: I4e00f2e2289c1381f0238d2a4acb606a0ab551a9
Reported-by:  <katao@xiaomi.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@google.com>