selftests: ncdevmem: make chunking optional

Add new -z argument to specify max IOV size. By default, use
single large IOV.

Signed-off-by: Stanislav Fomichev <stfomichev@gmail.com>
Reviewed-by: Mina Almasry <almasrymina@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>