nvmet-tcp: Don't set the request's data_len

It's not apprporiate for the transports to set the data_len
field of the request which is only used by the core.

In this case, just use a variable on the stack to store the
length of the sgl for comparison.

Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Logan Gunthorpe <logang@deltatee.com>
Signed-off-by: Keith Busch <kbusch@kernel.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>