[iscsi] Limit maximum transfer size to MaxBurstLength

[iscsi] Limit maximum transfer size to MaxBurstLength

We currently specify only the iSCSI default value for MaxBurstLength
and ignore any negotiated value, since our internal block device API
allows only for receiving directly into caller-allocated buffers and
so we have no intrinsic limit on burst length.

A conscientious target may however refuse to attempt a transfer that
we request for a number of blocks that would exceed the negotiated
maximum burst length.

Fix by recording the negotiated maximum burst length and using it to
limit the maximum number of blocks per transfer as reported by the
SCSI layer.

Signed-off-by: Michael Brown <mcb30@ipxe.org>