tests: do not malloc on bdev write

For page-sized allocations, it is better to use alloc_page than it is to use
malloc. The reason is that malloc for that size will arrive at malloc_large,
which is a locked operation, while alloc_page will proceed locklessly if there
is room in the per-cpu buffers.

Although it is just a test, since the goal is to saturate the disk, doing so
will allow us to get a closer picture, since the completion handler won't
block.

In KVM with my weird disk I see now ~65 Mbps where I previously saw ~45Mbps.
Interestingly enough, it doesn't seem to make a whole lot of difference for
Xen. There is a difference, but it is not nowhere as near.

Reviewed-by: Tomasz Grabiec <tgrabiec@gmail.com>
Signed-off-by: Glauber Costa <glommer@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>