f2fs: fix to avoid potential race on sbi->unusable_block_count access/update

Use sbi.stat_lock to protect sbi->unusable_block_count accesss/udpate, in
order to avoid potential race on it.

Signed-off-by: Chao Yu <[email protected]>
Signed-off-by: Jaegeuk Kim <[email protected]>
3 files changed