OSV-2021-578 Stack-use-after-return in v9fs_string_free

OSS-Fuzz report: https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=32624 Crash type: Stack-use-after-return READ 8 Crash state: v9fsstringfree v9fsxattrwalk coroutinetrampoline...