util: Avoid waste space when linear alloc'ing large sizes

util: Avoid waste space when linear alloc'ing large sizes

In the linear allocator, when a size larger than the minimum
buffer size is allocated, we currently create the new buffer
to fit exactly the requested size.

In that case, don't bother updating the `latest` pointer, since
this newly created buffer is already full.  In the worst case,
the current `latest` is full and it would be the same; in the
best case, there's still room for small allocations, so we avoid
wasting space if the next allocations would fit.

Reviewed-by: Emma Anholt <emma@anholt.net>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/25517>