util/vfio-helpers: Use g_file_read_link()

When _FORTIFY_SOURCE=2, glibc version is 2.35, and GCC version is
12.1.0, the compiler complains as follows:

In file included from /usr/include/features.h:490,
                 from /usr/include/bits/libc-header-start.h:33,
                 from /usr/include/stdint.h:26,
                 from /usr/lib/gcc/aarch64-unknown-linux-gnu/12.1.0/include/stdint.h:9,
                 from /home/alarm/q/var/qemu/include/qemu/osdep.h:94,
                 from ../util/vfio-helpers.c:13:
In function 'readlink',
    inlined from 'sysfs_find_group_file' at ../util/vfio-helpers.c:116:9,
    inlined from 'qemu_vfio_init_pci' at ../util/vfio-helpers.c:326:18,
    inlined from 'qemu_vfio_open_pci' at ../util/vfio-helpers.c:517:9:
/usr/include/bits/unistd.h:119:10: error: argument 2 is null but the corresponding size argument 3 value is 4095 [-Werror=nonnull]
  119 |   return __glibc_fortify (readlink, __len, sizeof (char),
      |          ^~~~~~~~~~~~~~~

This error implies the allocated buffer can be NULL. Use
g_file_read_link(), which allocates buffer automatically to avoid the
error.

Signed-off-by: Akihiko Odaki <akihiko.odaki@daynix.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Cédric Le Goater <clg@redhat.com>
Signed-off-by: Cédric Le Goater <clg@redhat.com>
(cherry picked from commit dbdea0d)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>

util/vfio-helpers.c

@@ -106,15 +106,17 @@ struct QEMUVFIOState {
  */
 static char *sysfs_find_group_file(const char *device, Error **errp)
 {
+    g_autoptr(GError) gerr = NULL;
     char *sysfs_link;
     char *sysfs_group;
     char *p;
     char *path = NULL;
 
     sysfs_link = g_strdup_printf("/sys/bus/pci/devices/%s/iommu_group", device);
-    sysfs_group = g_malloc0(PATH_MAX);
-    if (readlink(sysfs_link, sysfs_group, PATH_MAX - 1) == -1) {
-        error_setg_errno(errp, errno, "Failed to find iommu group sysfs path");
+    sysfs_group = g_file_read_link(sysfs_link, &gerr);
+    if (gerr) {
+        error_setg(errp, "Failed to find iommu group sysfs path: %s",
+                   gerr->message);
         goto out;
     }
     p = strrchr(sysfs_group, '/');