Fix StallWrite crash with mixed of slowdown/no_slowdown writes (#7508)

Fix StallWrite crash with mixed of slowdown/no_slowdown writes (#7508)

Summary:
`BeginWriteStall()` removes no_slowdown write from the write
list and updates `link_newer`, which makes `CreateMissingNewerLinks()`
thought all write list has valid `link_newer` and failed to create link
for all writers.
It caused flaky test and SegFault for release build.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/7508

Test Plan: Add unittest to reproduce the issue.

Reviewed By: anand1976

Differential Revision: D24126601

Pulled By: jay-zhuang

fbshipit-source-id: f8ac5dba653f7ee1b0950296427d4f5f8ee34a06