generic/551: Adjust the size argument for aio-dio-write-verify

generic/551: Adjust the size argument for aio-dio-write-verify

generic/551 sometimes fails because it's killed by OOM killer.
That is because aio-dio-write-verify, which is called by
generic/551, tries to allocate memory even though the total
allocation size exceeds the available memory.

aio-dio-write-verify allocates memory according to the 'size'
argument, and generic/551 passes the argument.

Stop adding the argument when the total size is exceeds
the available memory.

Signed-off-by: Masayoshi Mizuma <m.mizuma@jp.fujitsu.com>
Reviewed-by: Eryu Guan <guaneryu@gmail.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>