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>

Documentation/networking/pktgen.txt

@@ -145,6 +145,7 @@ Examples:
                               UDPCSUM,
                               IPSEC # IPsec encapsulation (needs CONFIG_XFRM)
                               NODE_ALLOC # node specific memory allocation
+                              NO_TIMESTAMP # disable timestamping
 
  pgset spi SPI_VALUE     Set specific SA used to transform packet.
 
@@ -287,6 +288,7 @@ flag
   UDPCSUM
   IPSEC
   NODE_ALLOC
+  NO_TIMESTAMP
 
 dst_min
 dst_max

net/core/pktgen.c

@@ -1267,6 +1267,9 @@ static ssize_t pktgen_if_write(struct file *file,
 		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",