Summary:
Add assertions to check direct IO's alignment requirements in MultiRead.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6739

Test Plan: make check

Reviewed By: siying

Differential Revision: D21143825

Pulled By: cheng-chang

fbshipit-source-id: 26f1623b062a1851080771128feac0669a61f5e9