iscsi: Rely on block layer to break up large requests

iscsi: Rely on block layer to break up large requests

Now that the block layer honors max_request, we don't need to
bother with an EINVAL on overlarge requests, but can instead
assert that requests are well-behaved.

Signed-off-by: Eric Blake <[email protected]>
Reviewed-by: Fam Zheng <[email protected]>
Reviewed-by: Stefan Hajnoczi <[email protected]>
Acked-by: Paolo Bonzini <[email protected]>
Message-id: 1468607524[email protected]
Signed-off-by: Stefan Hajnoczi <[email protected]>