vfio-pci: Fix buffer overfill

A read from a range hidden from the user (ex. MSI-X vector table)
attempts to fill the user buffer up to the end of the excluded range
instead of up to the requested count.  Fix it.

Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
Cc: stable@vger.kernel.org
diff --git a/drivers/vfio/pci/vfio_pci_rdwr.c b/drivers/vfio/pci/vfio_pci_rdwr.c
index 4362d9e..f72323e 100644
--- a/drivers/vfio/pci/vfio_pci_rdwr.c
+++ b/drivers/vfio/pci/vfio_pci_rdwr.c
@@ -240,17 +240,17 @@
 			filled = 1;
 		} else {
 			/* Drop writes, fill reads with FF */
+			filled = min((size_t)(x_end - pos), count);
 			if (!iswrite) {
 				char val = 0xFF;
 				size_t i;
 
-				for (i = 0; i < x_end - pos; i++) {
+				for (i = 0; i < filled; i++) {
 					if (put_user(val, buf + i))
 						goto out;
 				}
 			}
 
-			filled = x_end - pos;
 		}
 
 		count -= filled;