net: check for TUNSETOFFLOAD support before trying to enable offload features

This avoids the "TUNSETOFFLOAD ioctl() failed: Invalid argument" message
on kernels without TUNSETOFFLOAD support.
Signed-off-by: NPierre Riteau <Pierre.Riteau@irisa.fr>
Signed-off-by: NMark McLoughlin <markmc@redhat.com>
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>

net/tap-linux.c

@@ -131,6 +131,11 @@ void tap_fd_set_offload(int fd, int csum, int tso4,
 {
     unsigned int offload = 0;
 
+    /* Check if our kernel supports TUNSETOFFLOAD */
+    if (ioctl(fd, TUNSETOFFLOAD, 0) != 0 && errno == EINVAL) {
+        return;
+    }
+
     if (csum) {
         offload |= TUN_F_CSUM;
         if (tso4)