f2fs: Fix the typos in comments

This patch fixes minor typos in comments in f2fs.

Signed-off-by: Swarna Prabhu <s.prabhu@samsung.com>
Reviewed-by: Luis Chamberlain <mcgrof@kernel.org>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>