vfio: Fix overrun after readlink() fills buffer completely

readlink() returns the number of bytes written to the buffer, and it
doesn't write a terminating null byte.  vfio_init() writes it itself.
Overruns the buffer when readlink() filled it completely.

Fix by treating readlink() filling the buffer completely as error,
like we do in pci-assign.c's assign_failed_examine().

Spotted by Coverity.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>

hw/misc/vfio.c

@@ -3681,10 +3681,10 @@ static int vfio_initfn(PCIDevice *pdev)
 
     strncat(path, "iommu_group", sizeof(path) - strlen(path) - 1);
 
-    len = readlink(path, iommu_group_path, PATH_MAX);
-    if (len <= 0) {
+    len = readlink(path, iommu_group_path, sizeof(path));
+    if (len <= 0 || len >= sizeof(path)) {
         error_report("vfio: error no iommu_group for device");
-        return -errno;
+        return len < 0 ? -errno : ENAMETOOLONG;
     }
 
     iommu_group_path[len] = 0;