pktgen: adjust flag NO_TIMESTAMP to be more pktgen compliant

Allow flag NO_TIMESTAMP to turn timestamping on again, like other flags,
with a negation of the flag like !NO_TIMESTAMP.

Also document the option flag NO_TIMESTAMP.

Fixes: afb84b626184 ("pktgen: add flag NO_TIMESTAMP to disable timestamping")
Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/core/pktgen.c b/net/core/pktgen.c
index 508155b..43bb215 100644
--- a/net/core/pktgen.c
+++ b/net/core/pktgen.c
@@ -1267,6 +1267,9 @@
 		else if (strcmp(f, "NO_TIMESTAMP") == 0)
 			pkt_dev->flags |= F_NO_TIMESTAMP;
 
+		else if (strcmp(f, "!NO_TIMESTAMP") == 0)
+			pkt_dev->flags &= ~F_NO_TIMESTAMP;
+
 		else {
 			sprintf(pg_result,
 				"Flag -:%s:- unknown\nAvailable flags, (prepend ! to un-set flag):\n%s",