f2fs: don't allow negative ->write_io_size_bits

As Dan reported:

"We put an upper bound on ->write_io_size_bits but we don't have a lower
bound."

So let's add lower bound check for ->write_io_size_bits in parse_options().

[We don't allow configuring ->write_io_size_bits to zero, since at least
we need to fill one dummy page for aligned IO.]
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/super.c

@@ -586,7 +586,7 @@ static int parse_options(struct super_block *sb, char *options)
 		case Opt_io_size_bits:
 			if (args->from && match_int(args, &arg))
 				return -EINVAL;
-			if (arg > __ilog2_u32(BIO_MAX_PAGES)) {
+			if (arg <= 0 || arg > __ilog2_u32(BIO_MAX_PAGES)) {
 				f2fs_msg(sb, KERN_WARNING,
 					"Not support %d, larger than %d",
 					1 << arg, BIO_MAX_PAGES);