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

diff --git a/net/core/dev_ioctl.c b/net/core/dev_ioctl.c
index 5b7d0e1..cf999e0 100644
--- a/net/core/dev_ioctl.c
+++ b/net/core/dev_ioctl.c
@@ -327,6 +327,7 @@
 		    cmd == SIOCBRADDIF ||
 		    cmd == SIOCBRDELIF ||
 		    cmd == SIOCSHWTSTAMP ||
+		    cmd == SIOCGHWTSTAMP ||
 		    cmd == SIOCWANDEV) {
 			err = -EOPNOTSUPP;
 			if (ops->ndo_do_ioctl) {
@@ -546,6 +547,7 @@
 	 */
 	default:
 		if (cmd == SIOCWANDEV ||
+		    cmd == SIOCGHWTSTAMP ||
 		    (cmd >= SIOCDEVPRIVATE &&
 		     cmd <= SIOCDEVPRIVATE + 15)) {
 			dev_load(net, ifr.ifr_name);