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 4096 bytes explicitly.

Bug: 76407663
Change-Id: I3392646648264ad1ca78e4b87240edc9385a0cc4
Reported-by:  <katao@xiaomi.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@google.com>