MDEV-38419: Fix some clang-21 -fsanitize=memory failures

[dr-m]

• The Jira issue number for this PR is: MDEV-38419

Description

clang++-21 -fsanitize=memory is flagging more errors than its predecessors. Here I am trying to fix some root causes of it, by making our code prettier. As far as I can tell, this is not fixing any actual cases of memory being uninitialized, only making the code a little simpler.
fil_space_t::free_page(): Turn a parameter into a template parameter, and remove some duplicated code. This fixes an error that was flagged by clang++-21 -fsanitize=memory in buf_page_create().

fseg_create(): Merge the parameter reserved_extent to n_reserved. This fixes an error about n_reserved being uninitialized.

fseg_alloc_free_page_low(): Simplify a debug assertion.

flst_add_last(), flst_remove(): Reduce the scope of a conditionally initialized variable.

Even after these fixes, the server would fail to bootstrap with this build variant. I got a little further by adding

#if defined __clang__ && __has_feature(memory_sanitizer)
__attribute__((optnone)) // Work around an issue with clang-21
#endif

to several functions. But I don’t think this is the correct solution; we must file a clang bug report, with some reduced input that reproduces the issue.

Release Notes

N/A

How can this PR be tested?

cmake -DWITH_MSAN=ON build using clang-21, and debug the bootstrap failures with mysql-test/mtr --boot-rr main.1st.

Basing the PR against the correct MariaDB version

• This is a new feature or a refactoring, and the PR is based against the main branch.
• This is a bug fix, and the PR is based against the earliest maintained branch in which the bug can be reproduced.

PR quality check

• I checked the CODING_STANDARDS.md file and my PR conforms to this where appropriate.
• For any trivial modifications to the PR, I am ok with the reviewer making the changes themselves.

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