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.
Reviewed by: Thirunarayanan Balathandayuthapani