fsck.f2fs: fix alignment on multi-partition support

fsck.f2fs: fix alignment on multi-partition support

- mkfs.f2fs -s 4 -c second_dev first_dev
- fsck.f2fs first

Info: Segments per section = 1
Info: Sections per zone = 1
Info: sector size = 512
Info: total sectors = 69668488 (34017 MB)
Segment count (19128) mismatch with total segments from devices (19130)	Can't find a valid F2FS superblock at 0x0
Segment count (19128) mismatch with total segments from devices (19130)	Can't find a valid F2FS superblock at 0x1

Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>