net: enable timestamp static key if CPU

For systems that use CPU isolation (via nohz_full), creating or destroying a socket with SO_TIMESTAMP, SO_TIMESTAMPNS or SO_TIMESTAMPING with flag SOF_TIMESTAMPING_RX_SOFTWARE will cause a static key to be enabled/disabled. This in turn causes undesired IPIs to isolated CPUs.

So enable the static key unconditionally, if CPU isolation is enabled, thus avoiding the IPIs.

---
v2: mention SOF_TIMESTAMPING_OPT_TX_SWHW in the commit log (Willem de Bruijn / Paolo Abeni)
v3: SOF_TIMESTAMPING_OPT_TX_SWHW is irrelevant (Willem de Bruijn)
v4: additional changelog improvements (Willem de Bruijn)
v5: late initcall not necessary, can use subsys initcall (Willem de Bruijn)
v6: rebase against net-next, change subject (Jakub Kicinski)
v7: change location of linux/sched #include (Jakub Kicinski)

index 5d36a634f468..718a00e70d53 100644
@@ -78,6 +78,7 @@
 #include <linux/slab.h>
 #include <linux/sched.h>
 #include <linux/sched/mm.h>
+#include <linux/sched/isolation.h>
 #include <linux/smpboot.h>
 #include <linux/mutex.h>
 #include <linux/rwsem.h>

@@ -11890,6 +11891,10 @@ static int __init net_dev_init(void)
 				       NULL, dev_cpu_dead);
 	WARN_ON(rc < 0);
 	rc = 0;
+
+	/* avoid static key IPIs to isolated CPUs */
+	if (housekeeping_enabled(HK_TYPE_MISC))
+		net_enable_timestamp();
 out:
 	if (rc < 0) {
 		for_each_possible_cpu(i) {