vhost-net: enable zerocopy tx by default

Zero copy TX has been around for a while now.
We seem to be down to eliminating theoretical bugs
and performance tuning at this point:
it's probably time to enable it by default so that
most users get the benefit.

Keep the flag around meanwhile so users can experiment
with disabling this if they experience regressions.
I expect that we will remove it in the future.
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

drivers/vhost/net.c

@@ -30,9 +30,10 @@
 
 #include "vhost.h"
 
-static int experimental_zcopytx;
+static int experimental_zcopytx = 1;
 module_param(experimental_zcopytx, int, 0444);
-MODULE_PARM_DESC(experimental_zcopytx, "Enable Experimental Zero Copy TX");
+MODULE_PARM_DESC(experimental_zcopytx, "Enable Zero Copy TX;"
+		                       " 1 -Enable; 0 - Disable");
 
 /* Max number of bytes transferred before requeueing the job.
  * Using this limit prevents one virtqueue from starving others. */