affs: fix potential memory leak when parsing option 'prefix'

When specifying option 'prefix' multiple times, current option parsing
will cause memory leak. Hence, call kfree for previous one in this
case.

Signed-off-by: Chengguang Xu <[e-mail]>
Signed-off-by: David Sterba <[e-mail]>

fs/affs/super.c

···
 			affs_set_opt(*mount_opts, SF_NO_TRUNCATE);
 			break;
 		case Opt_prefix:
+			kfree(*prefix);
 			*prefix = match_strdup(&args[0]);
 			if (!*prefix)
 				return 0;