Remove explicit padding from CacheAlignedInstrumentedMutex (#9809)

Remove explicit padding from CacheAlignedInstrumentedMutex (#9809)

Summary:
Fixes https://github.com/facebook/rocksdb/issues/9779.

The padding at the end of a struct is added implicitly according to the
sizeof spec: "When applied to a class, the result is the
number of bytes in an object of that class including any padding
required for placing objects of that type in an array"
(https://eel.is/c++draft/expr.sizeof#2.sentence-2). We should drop the
explicit padding since it assumed support for zero-length arrays, which
is non-standard.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/9809

Test Plan: rely on CI

Reviewed By: riversand963

Differential Revision: D35413496

Pulled By: ajkr

fbshipit-source-id: 25d52ca45e648ad0d5657149f26f6adecbed1cb4