block: test-iosched: fix bio allocation for test requests

Unit tests submit large requests of 512KB made of 128 bios.
Current allocation was done via kmalloc which may not be able
to allocate such a large buffer which is also physically contiguous.
Using kmalloc to allocate each bio separately is also problematic as
it might not be page aligned. Some bio may end up using more than a
single memory segment which will fail the mapping of the bios to
the request which supports up to 128 physical segments.
To avoid such failure, allocate a separate page for each bio
(bio size is single page size).

Change-Id: Id0da394d458942e093d924bc7aa23aa3231cdca7
Signed-off-by: Gilad Broner <gbroner@codeaurora.org>