check_f2fs: fix wrong calculation of # of sectors Bug: 279519292 Merged-In: Ief3718f35f8ad10bc3b2efc2e1d3445016ee20da Change-Id: Ief3718f35f8ad10bc3b2efc2e1d3445016ee20da Signed-off-by: Jaegeuk Kim <[email protected]>