NVMe: check for integer overflow in nvme_map_user_pages()

You need to have CAP_SYS_ADMIN to trigger this overflow but it makes the
static checkers complain so we should fix it.  The worry is that
"length" comes from copy_from_user() so we need to check that "length +
offset" can't overflow.

I also changed the min_t() cast to be unsigned instead of signed.  Now
that we cap "length" to INT_MAX it doesn't make a difference, but it's a
little easier for reviewers to know that large values aren't cast to
negative.
Signed-off-by: NDan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: NMatthew Wilcox <matthew.r.wilcox@intel.com>

drivers/block/nvme-core.c

@@ -1206,7 +1206,7 @@ struct nvme_iod *nvme_map_user_pages(struct nvme_dev *dev, int write,
 
 	if (addr & 3)
 		return ERR_PTR(-EINVAL);
-	if (!length)
+	if (!length || length > INT_MAX - PAGE_SIZE)
 		return ERR_PTR(-EINVAL);
 
 	offset = offset_in_page(addr);
@@ -1227,7 +1227,8 @@ struct nvme_iod *nvme_map_user_pages(struct nvme_dev *dev, int write,
 	sg_init_table(sg, count);
 	for (i = 0; i < count; i++) {
 		sg_set_page(&sg[i], pages[i],
-				min_t(int, length, PAGE_SIZE - offset), offset);
+			    min_t(unsigned, length, PAGE_SIZE - offset),
+			    offset);
 		length -= (PAGE_SIZE - offset);
 		offset = 0;
 	}