f2fs: fix typo

[ Upstream commit d382e36970ecf8242921400db2afde15fb6ed49e ]

Fix typo in f2fs.h
Detected by Jaeyoon Choi

Signed-off-by: Yonggil Song <yonggil.song@samsung.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>
Stable-dep-of: aaf8c0b9ae04 ("f2fs: reduce expensive checkpoint trigger frequency")
Signed-off-by: Sasha Levin <sashal@kernel.org>