Summary:
In `SaveValue()`, the read lock needs to be obtained before `VerifyEntryChecksum()` because the KV checksum verification reads the entire value metadata+data, which is all mutable when `ColumnFamilyOptions::inplace_update_support == true`.
In `MemTable::Update()`, the write lock needs to be obtained before mutating the value metadata (changing the value size) because it can be read concurrently.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/12624
Test Plan:
```
$ make COMPILE_WITH_TSAN=1 -j56 db_stress
...
$ python3 tools/db_crashtest.py blackbox --simple --max_key=10 --inplace_update_support=1 --interval=10 --allow_concurrent_memtable_write=0
```
Reviewed By: cbi42
Differential Revision: D57034571
Pulled By: ajkr
fbshipit-source-id: 3dddf881ad87923143acdf6bfec12ce47bb13a48
