Fix for logio header that doesn't fit in one page due to chunk size

[MichaelBrim]

Description

Avoids unexpected unifyfs_mount() failures for clients that customize UNIFYFS_LOGIO_CHUNK_SIZE.

Motivation and Context

The current logio header uses a single page to hold its metadata, which includes the chunk slotmap. For large shmem or spill regions with small chunk sizes, it is possible that a page is not large enough to hold the whole slotmap, which causes client applications to fail during unifyfs_mount().

This PR allows for the logio header to grow to as many pages as necessary to hold the chunk slotmap for the given region and chunk size.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Performance enhancement (non-breaking change which improves efficiency)
• Code cleanup (non-breaking change which makes code smaller or more readable)
• Breaking change (fix or feature that would cause existing functionality to change)
• Testing (addition of new tests or update to current tests)
• Documentation (a change to man pages or other documentation)

Checklist:

• My code follows the UnifyFS code style requirements.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.
• All commit messages are properly formatted.

[CamStan]

Yay, more CI problems. 😫
I rebuilt the dev branch in my fork, and Travis is failing there as well. So your changes aren't causing anything.

At first glance the only difference I could see between previous successful builds was the Mercury version had changed, but changing that still fails for the same reason.

So I starting thinking something changed with Spack.
Wondering if this is causing the problem spack/spack#24020

[CamStan]

The issue in Spack got resolved and fixed our Travis builds.

[adammoody]

Thanks @MichaelBrim and @CamStan