vduse: Use fixed 4KB bounce pages for non-4KB page size

vduse: Use fixed 4KB bounce pages for non-4KB page size

The allocation granularity of bounce pages is PAGE_SIZE. This may cause
even small IO requests to occupy an entire bounce page exclusively. The
kind of memory waste will be more significant when PAGE_SIZE is larger
than 4KB (e.g. arm64 with 64KB pages).

So, optimize it by using fixed 4KB bounce maps and iova allocation
granularity. A single IO request occupies at least a 4KB bounce page
instead of the entire memory page of PAGE_SIZE.

Signed-off-by: Sheng Zhao <sheng.zhao@bytedance.com>
Message-Id: <20250925113516.60305-1-sheng.zhao@bytedance.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>