FreeBSD: unregister mountroot eventhandler on unload

Otherwise if zfs is unloaded and reroot is being used it trips over a
stale pointer.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Sponsored by:	Rubicon Communications, LLC ("Netgate")
Signed-off-by: Mateusz Guzik <mjguzik@gmail.com>
Closes openzfs#16242

module/os/freebsd/zfs/kmod_core.c

@@ -111,6 +111,7 @@ static int zfs__fini(void);
 static void zfs_shutdown(void *, int);
 
 static eventhandler_tag zfs_shutdown_event_tag;
+static eventhandler_tag zfs_mountroot_event_tag;
 
 #define	ZFS_MIN_KSTACK_PAGES 4
 
@@ -305,16 +306,25 @@ zfs_modevent(module_t mod, int type, void *unused __unused)
 	switch (type) {
 	case MOD_LOAD:
 		err = zfs__init();
-		if (err == 0)
+		if (err == 0) {
 			zfs_shutdown_event_tag = EVENTHANDLER_REGISTER(
 			    shutdown_post_sync, zfs_shutdown, NULL,
 			    SHUTDOWN_PRI_FIRST);
+			zfs_mountroot_event_tag = EVENTHANDLER_REGISTER(
+			    mountroot, spa_boot_init, NULL,
+			    SI_ORDER_ANY);
+		}
 		return (err);
 	case MOD_UNLOAD:
 		err = zfs__fini();
-		if (err == 0 && zfs_shutdown_event_tag != NULL)
-			EVENTHANDLER_DEREGISTER(shutdown_post_sync,
-			    zfs_shutdown_event_tag);
+		if (err == 0) {
+			if (zfs_shutdown_event_tag != NULL)
+				EVENTHANDLER_DEREGISTER(shutdown_post_sync,
+				    zfs_shutdown_event_tag);
+			if (zfs_mountroot_event_tag != NULL)
+				EVENTHANDLER_DEREGISTER(mountroot,
+				    zfs_mountroot_event_tag);
+		}
 		return (err);
 	case MOD_SHUTDOWN:
 		return (0);
@@ -330,9 +340,6 @@ static moduledata_t zfs_mod = {
 	0
 };
 
-#ifdef _KERNEL
-EVENTHANDLER_DEFINE(mountroot, spa_boot_init, NULL, 0);
-#endif
 
 FEATURE(zfs, "OpenZFS support");