gve: Implement gettimex64 with -EOPNOTSUPP

gve implemented a ptp_clock for sole use of do_aux_work at this time.
ptp_clock_gettime() and ptp_sys_offset() assume every ptp_clock has
implemented either gettimex64 or gettime64. Stub gettimex64 and return
-EOPNOTSUPP to prevent NULL dereferencing.

Fixes: acd1638 ("gve: Add initial PTP device support")
Reported-by: syzbot+c8c0e7ccabd456541612@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=c8c0e7ccabd456541612
Signed-off-by: Tim Hostetler <thostet@google.com>
Reviewed-by: Harshitha Ramamurthy <hramamurthy@google.com>
Reviewed-by: Kuniyuki Iwashima <kuniyu@google.com>
Signed-off-by: Joshua Washington <joshwash@google.com>
Link: https://patch.msgid.link/20251029184555.3852952-2-joshwash@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: tdh911

drivers/net/ethernet/google/gve/gve_ptp.c

@@ -26,6 +26,13 @@ int gve_clock_nic_ts_read(struct gve_priv *priv)
 	return 0;
 }
 
+static int gve_ptp_gettimex64(struct ptp_clock_info *info,
+			      struct timespec64 *ts,
+			      struct ptp_system_timestamp *sts)
+{
+	return -EOPNOTSUPP;
+}
+
 static long gve_ptp_do_aux_work(struct ptp_clock_info *info)
 {
 	const struct gve_ptp *ptp = container_of(info, struct gve_ptp, info);
@@ -47,6 +54,7 @@ static long gve_ptp_do_aux_work(struct ptp_clock_info *info)
 static const struct ptp_clock_info gve_ptp_caps = {
 	.owner          = THIS_MODULE,
 	.name		= "gve clock",
+	.gettimex64	= gve_ptp_gettimex64,
 	.do_aux_work	= gve_ptp_do_aux_work,
 };