mmap-cache: enforce an unused windows minimum

mmap-cache: enforce an unused windows minimum

With many fds the global windows count generally exceeds the
minimum.   This results in always reusing the unused entry if
there is one, which becomes a sort of degenerate case where we're
just constantly unmapping->mapping.

Instead let's try always have at least several unused windows on
the unused list before we resort to churning through it.

Fixes #34516