util: Fix regression of wwn reading

Introduced by commit 244ce462, which refactored the helper for wwn
reading, however, it forgot to change the old "strndup" and "sizeof(buf)",
"sizeof(buf)" operates on the fixed length array ("buf") in the old code,
but now "buf" is a pointer.

Before the fix:

% virsh nodedev-dumpxml scsi_host5
<device>
  <name>scsi_host5</name>
  <parent>pci_0000_04_00_1</parent>
  <capability type='scsi_host'>
    <host>5</host>
    <capability type='fc_host'>
      <wwnn>2001001b</wwnn>
      <wwpn>2101001b</wwpn>
      <fabric_wwn>2001000d</fabric_wwn>
    </capability>
  </capability>
</device>

With the fix:

% virsh nodedev-dumpxml scsi_host5
<device>
  <name>scsi_host5</name>
  <parent>pci_0000_04_00_1</parent>
  <capability type='scsi_host'>
    <host>5</host>
    <capability type='fc_host'>
      <wwnn>0x2001001b32a9da4e</wwnn>
      <wwpn>0x2101001b32a9da4e</wwpn>
      <fabric_wwn>0x2001000dec9877c1</fabric_wwn>
    </capability>
  </capability>
</device>

src/util/virutil.c

@@ -1694,10 +1694,8 @@ virReadFCHost(const char *sysfs_prefix,
     else
         p = buf;
 
-    if (!(*result = strndup(p, sizeof(buf)))) {
-        virReportOOMError();
+    if (VIR_STRDUP(*result, p) < 0)
         goto cleanup;
-    }
 
     ret = 0;
 cleanup: