Register .remount_fs handler

Register the missing .remount_fs handler.  This handler isn't strictly
required because the VFS does a pretty good job updating most of the
MS_* flags.  However, there's no harm in using the hook to call the
registered zpl callback for various MS_* flags.  Additionaly, this
allows us to lay the ground work for more complicated argument parsing
in the future.

module/zfs/zpl_super.c

@@ -129,6 +129,16 @@ zpl_statfs(struct dentry *dentry, struct kstatfs *statp)
 	return (error);
 }
 
+static int
+zpl_remount_fs(struct super_block *sb, int *flags, char *data)
+{
+	int error;
+	error = -zfs_remount(sb, flags, data);
+	ASSERT3S(error, <=, 0);
+
+	return (error);
+}
+
 static int
 zpl_show_options(struct seq_file *seq, struct vfsmount *vfsp)
 {
@@ -197,7 +207,7 @@ const struct super_operations zpl_super_operations = {
 	.freeze_fs	= NULL,
 	.unfreeze_fs	= NULL,
 	.statfs		= zpl_statfs,
-	.remount_fs	= NULL,
+	.remount_fs	= zpl_remount_fs,
 	.show_options	= zpl_show_options,
 	.show_stats	= NULL,
 };