nvmet_fc: fix better length checking

Reorganize nvmet_fc_handle_fcp_rqst() so that the nvmet req.transfer_len
field is set after the call nvmet_req_init(). An update to nvmet now
has nvmet_req_init() clearing the field, thus the fc transport was losing
the value.

Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: James Smart <james.smart@broadcom.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>