Make txg_wait_synced conditional in zfsvfs_teardown, for FreeBSD

This applies the same change in openzfs#9115 to FreeBSD.  This was actually the
old behavior in FreeBSD 12; it only regressed when FreeBSD support was
added to OpenZFS.  As far as I can tell, the timeline went like this:

* Illumos's zfsvfs_teardown used an unconditional txg_wait_synced
* Illumos added the dirty data check [^4]
* FreeBSD merged in Illumos's conditional check [^3]
* OpenZFS forked from Illumos
* OpenZFS removed the dirty data check in openzfs#7795 [^5]
* @mattmacy forked the OpenZFS repo and began to add FreeBSD support
* OpenZFS PR openzfs#9115[^1] recreated the same dirty data check that Illumos
  used, in slightly different form.  At this point the OpenZFS repo did
  not yet have multi-OS support.
* Matt Macy merged in FreeBSD support in openzfs#8987[^2] , but it was based on
  slightly outdated OpenZFS code.

In my local testing, this vastly improves the reboot speed of a server
with a large pool that has 1000 datasets and is resilvering an HDD.

[^1]: openzfs#9115
[^2]: openzfs#8987
[^3]: freebsd/freebsd-src@10b9d77
[^4]: illumos/illumos-gate@5aaeed5
[^5]: openzfs#7795

Sponsored by:	Axcient
Signed-off-by:	Alan Somers <asomers@gmail.com>

module/os/freebsd/zfs/zfs_vfsops.c

@@ -1649,9 +1649,18 @@ zfsvfs_teardown(zfsvfs_t *zfsvfs, boolean_t unmounting)
 	zfs_unregister_callbacks(zfsvfs);
 
 	/*
-	 * Evict cached data
+	 * Evict cached data. We must write out any dirty data before
+	 * disowning the dataset.
 	 */
-	if (!zfs_is_readonly(zfsvfs))
+	objset_t *os = zfsvfs->z_os;
+	boolean_t os_dirty = B_FALSE;
+	for (int t = 0; t < TXG_SIZE; t++) {
+		if (dmu_objset_is_dirty(os, t)) {
+			os_dirty = B_TRUE;
+			break;
+		}
+	}
+	if (!zfs_is_readonly(zfsvfs) && os_dirty)
 		txg_wait_synced(dmu_objset_pool(zfsvfs->z_os), 0);
 	dmu_objset_evict_dbufs(zfsvfs->z_os);
 	dd = zfsvfs->z_os->os_dsl_dataset->ds_dir;