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>

Documentation/networking/timestamping.txt

@@ -85,7 +85,7 @@ Filled in if SOF_TIMESTAMPING_SYS_HARDWARE is set. Requires support
 by the network device and will be empty without that support.
 
 
-SIOCSHWTSTAMP:
+SIOCSHWTSTAMP, SIOCGHWTSTAMP:
 
 Hardware time stamping must also be initialized for each device driver
 that is expected to do hardware time stamping. The parameter is defined in
@@ -115,6 +115,10 @@ Only a processes with admin rights may change the configuration. User
 space is responsible to ensure that multiple processes don't interfere
 with each other and that the settings are reset.
 
+Any process can read the actual configuration by passing this
+structure to ioctl(SIOCGHWTSTAMP) in the same way.  However, this has
+not been implemented in all drivers.
+
 /* possible values for hwtstamp_config->tx_type */
 enum {
 	/*
@@ -157,7 +161,8 @@ DEVICE IMPLEMENTATION
 
 A driver which supports hardware time stamping must support the
 SIOCSHWTSTAMP ioctl and update the supplied struct hwtstamp_config with
-the actual values as described in the section on SIOCSHWTSTAMP.
+the actual values as described in the section on SIOCSHWTSTAMP.  It
+should also support SIOCGHWTSTAMP.
 
 Time stamps for received packets must be stored in the skb. To get a pointer
 to the shared time stamp structure of the skb call skb_hwtstamps(). Then