Add explicit SI_SUB_EPOCH

Add explicit SI_SUB_EPOCH, after SI_SUB_TASKQ and before SI_SUB_SMP
(EARLY_AP_STARTUP).  Rename existing "SI_SUB_TASKQ + 1" to SI_SUB_EPOCH.

epoch(9) consumers cannot epoch_alloc() before SI_SUB_EPOCH:SI_ORDER_SECOND,
but likely should allocate before SI_SUB_SMP.  Prior to this change,
consumers (well, epoch itself, and net/if.c) just open-coded the
SI_SUB_TASKQ + 1 order to match epoch.c, but this was fragile.

Reviewed by:	mmacy
Differential Revision:	https://reviews.freebsd.org/D22503

sys/kern/subr_epoch.c

@@ -276,7 +276,7 @@ epoch_init(void *arg __unused)
 	global_epoch = epoch_alloc("Global", 0);
 	global_epoch_preempt = epoch_alloc("Global preemptible", EPOCH_PREEMPT);
 }
-SYSINIT(epoch, SI_SUB_TASKQ + 1, SI_ORDER_FIRST, epoch_init, NULL);
+SYSINIT(epoch, SI_SUB_EPOCH, SI_ORDER_FIRST, epoch_init, NULL);
 
 #if !defined(EARLY_AP_STARTUP)
 static void

sys/net/if.c

@@ -933,8 +933,7 @@ if_epochalloc(void *dummy __unused)
 	net_epoch_preempt = epoch_alloc("Net preemptible", EPOCH_PREEMPT);
 	net_epoch = epoch_alloc("Net", 0);
 }
-SYSINIT(ifepochalloc, SI_SUB_TASKQ + 1, SI_ORDER_ANY,
-    if_epochalloc, NULL);
+SYSINIT(ifepochalloc, SI_SUB_EPOCH, SI_ORDER_ANY, if_epochalloc, NULL);
 
 static void
 if_attachdomain(void *dummy)

sys/sys/kernel.h

@@ -124,6 +124,7 @@ enum sysinit_sub_id {
 	SI_SUB_MBUF		= 0x2700000,	/* mbuf subsystem */
 	SI_SUB_INTR		= 0x2800000,	/* interrupt threads */
 	SI_SUB_TASKQ		= 0x2880000,	/* task queues */
+	SI_SUB_EPOCH		= 0x2888000,	/* epoch subsystem */
 #ifdef EARLY_AP_STARTUP
 	SI_SUB_SMP		= 0x2900000,	/* start the APs*/
 #endif