mtd: core: Potential NULL dereference in mtd_otp_size()

If kmalloc() fails then it could lead to a NULL dereference.  Check and
return -ENOMEM on error.

Fixes: 4b361cf ("mtd: core: add OTP nvmem provider support")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Michael Walle <michael@walle.cc>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/YJ6Iw3iNvGycAWV6@mwanda

drivers/mtd/mtdcore.c

@@ -779,12 +779,16 @@ static void mtd_set_dev_defaults(struct mtd_info *mtd)
 
 static ssize_t mtd_otp_size(struct mtd_info *mtd, bool is_user)
 {
-	struct otp_info *info = kmalloc(PAGE_SIZE, GFP_KERNEL);
+	struct otp_info *info;
 	ssize_t size = 0;
 	unsigned int i;
 	size_t retlen;
 	int ret;
 
+	info = kmalloc(PAGE_SIZE, GFP_KERNEL);
+	if (!info)
+		return -ENOMEM;
+
 	if (is_user)
 		ret = mtd_get_user_prot_info(mtd, PAGE_SIZE, &retlen, info);
 	else