nfs_remount(): don't leak, don't ignore LSM options quietly

* if mount(2) passes something like "context=foo" with MS_REMOUNT
in flags (/sbin/mount.nfs will _not_ do that - you need to issue
the syscall manually), you'll get leaked copies for LSM options.
The reason is that instead of nfs_{alloc,free}_parsed_mount_data()
nfs_remount() uses kzalloc/kfree, which lacks the needed cleanup.

* selinux options are not changed on remount (as for any other
fs), but in case of NFS the failure is quiet - they are not compared
to what we used to have, with complaint in case of attempted changes.
Trivially fixed by converting to use of security_sb_remount().
Reviewed-by: David Howells <dhowells@redhat.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/nfs/super.c

@@ -2254,7 +2254,7 @@ nfs_remount(struct super_block *sb, int *flags, char *raw_data)
 					   options->version <= 6))))
 		return 0;
 
-	data = kzalloc(sizeof(*data), GFP_KERNEL);
+	data = nfs_alloc_parsed_mount_data();
 	if (data == NULL)
 		return -ENOMEM;
 
@@ -2293,8 +2293,10 @@ nfs_remount(struct super_block *sb, int *flags, char *raw_data)
 
 	/* compare new mount options with old ones */
 	error = nfs_compare_remount_data(nfss, data);
+	if (!error)
+		error = security_sb_remount(sb, &data->lsm_opts);
 out:
-	kfree(data);
+	nfs_free_parsed_mount_data(data);
 	return error;
 }
 EXPORT_SYMBOL_GPL(nfs_remount);