FreeBSD: Clean up zfsdev_close to match Linux

Resolve some oddities in zfsdev_close() which could result in a
panic and were not present in the equivalent function for Linux.

- Remove unused definition ZFS_MIN_MINOR
- FreeBSD: Simplify zfsdev state destruction
- Assert zs_minor is valid in zfsdev_close
- Make locking around zfsdev state match Linux

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Signed-off-by: Ryan Moeller <ryan@iXsystems.com>
Closes #11720

include/sys/zfs_ioctl.h

@@ -525,7 +525,6 @@ typedef struct zfs_useracct {
 } zfs_useracct_t;
 
 #define	ZFSDEV_MAX_MINOR	(1 << 16)
-#define	ZFS_MIN_MINOR	(ZFSDEV_MAX_MINOR + 1)
 
 #define	ZPOOL_EXPORT_AFTER_SPLIT 0x1
 

module/os/freebsd/zfs/kmod_core.c

@@ -182,23 +182,21 @@ zfsdev_ioctl(struct cdev *dev, ulong_t zcmd, caddr_t arg, int flag,
 static void
 zfsdev_close(void *data)
 {
-	zfsdev_state_t *zs, *zsp = data;
+	zfsdev_state_t *zs = data;
+
+	ASSERT(zs != NULL);
 
 	mutex_enter(&zfsdev_state_lock);
-	for (zs = zfsdev_state_list; zs != NULL; zs = zs->zs_next) {
-		if (zs == zsp)
-			break;
-	}
-	if (zs == NULL || zs->zs_minor <= 0) {
-		mutex_exit(&zfsdev_state_lock);
-		return;
-	}
+
+	ASSERT(zs->zs_minor != 0);
+
 	zs->zs_minor = -1;
 	zfs_onexit_destroy(zs->zs_onexit);
 	zfs_zevent_destroy(zs->zs_zevent);
-	mutex_exit(&zfsdev_state_lock);
 	zs->zs_onexit = NULL;
 	zs->zs_zevent = NULL;
+
+	mutex_exit(&zfsdev_state_lock);
 }
 
 static int