net_tstamp: Add SIOCGHWTSTAMP ioctl to match SIOCSHWTSTAMP

SIOCSHWTSTAMP returns the real configuration to the application
using it, but there is currently no way for any other
application to find out the configuration non-destructively.
Add a new ioctl for this, making it unprivileged.

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>

net/core/dev_ioctl.c

@@ -327,6 +327,7 @@ static int dev_ifsioc(struct net *net, struct ifreq *ifr, unsigned int cmd)
 		    cmd == SIOCBRADDIF ||
 		    cmd == SIOCBRDELIF ||
 		    cmd == SIOCSHWTSTAMP ||
+		    cmd == SIOCGHWTSTAMP ||
 		    cmd == SIOCWANDEV) {
 			err = -EOPNOTSUPP;
 			if (ops->ndo_do_ioctl) {
@@ -546,6 +547,7 @@ int dev_ioctl(struct net *net, unsigned int cmd, void __user *arg)
 	 */
 	default:
 		if (cmd == SIOCWANDEV ||
+		    cmd == SIOCGHWTSTAMP ||
 		    (cmd >= SIOCDEVPRIVATE &&
 		     cmd <= SIOCDEVPRIVATE + 15)) {
 			dev_load(net, ifr.ifr_name);