f2fs: replace si->sbi w/ sbi in stat_show()

For each loop add a local f2fs_sb_info pointer insted of looking it up.

Signed-off-by: Yangtao Li <frank.li@vivo.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>