Make txg_wait_synced conditional in zfsvfs_teardown

The call to txg_wait_synced in zfsvfs_teardown should
be made conditional on the objset having dirty data.
This can prevent unnecessary txg_wait_synced during
some unmount operations.

Reviewed-by: Matt Ahrens <matt@delphix.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Paul Zuchowski <pzuchowski@datto.com>
Closes #9115

Author: PaulZ-98

module/zfs/zfs_vfsops.c

@@ -1778,8 +1778,17 @@ zfsvfs_teardown(zfsvfs_t *zfsvfs, boolean_t unmounting)
 	 * 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);
 
 	return (0);