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: NChengguang Xu <cgxu519@gmx.com>
Signed-off-by: NDavid Sterba <dsterba@suse.com>

fs/affs/super.c

@@ -241,6 +241,7 @@ parse_options(char *options, kuid_t *uid, kgid_t *gid, int *mode, int *reserved,
 			affs_set_opt(*mount_opts, SF_NO_TRUNCATE);
 			break;
 		case Opt_prefix:
+			kfree(*prefix);
 			*prefix = match_strdup(&args[0]);
 			if (!*prefix)
 				return 0;