iommufd: Don't overflow during division for dirty tracking

If pgshift is 63 then BITS_PER_TYPE(*bitmap->bitmap) * pgsize will overflow
to 0 and this triggers divide by 0.

In this case the index should just be 0, so reorganize things to divide
by shift and avoid hitting any overflows.

Link: https://patch.msgid.link/r/0-v1-663679b57226+172-iommufd_dirty_div0_jgg@nvidia.com
Cc: stable@vger.kernel.org
Fixes: 58ccf01 ("vfio: Add an IOVA bitmap support")
Reviewed-by: Joao Martins <joao.m.martins@oracle.com>
Reviewed-by: Nicolin Chen <nicolinc@nvidia.com>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Reported-by: syzbot+093a8a8b859472e6c257@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=093a8a8b859472e6c257
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>

Author: jgunthorpe

drivers/iommu/iommufd/iova_bitmap.c

@@ -130,9 +130,8 @@ struct iova_bitmap {
 static unsigned long iova_bitmap_offset_to_index(struct iova_bitmap *bitmap,
 						 unsigned long iova)
 {
-	unsigned long pgsize = 1UL << bitmap->mapped.pgshift;
-
-	return iova / (BITS_PER_TYPE(*bitmap->bitmap) * pgsize);
+	return (iova >> bitmap->mapped.pgshift) /
+	       BITS_PER_TYPE(*bitmap->bitmap);
 }
 
 /*