f2fs: set zstd compress level correctly

Fixes: cf30f6a5 ("lib: zstd: Add kernel-specific API")
Signed-off-by: Sheng Yong <shengyong@oppo.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Reviewed-by: Nick Terrell <terrelln@fb.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>