f2fs: clean up w/ sbi->log_sectors_per_block

Use sbi->log_sectors_per_block to clean up below calculated one:

unsigned int log_sectors_per_block = sbi->log_blocksize - SECTOR_SHIFT;

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