nvmet-tcp: fix possible list corruption for unexpected command failure

nvmet_tcp_handle_req_failure needs to understand weather to prepare
for incoming data or the next pdu. However if we misidentify this, we
will wait for 0-length data, and queue the response although nvmet_req_init
already did that.

The particular command was namespace management command with no data,
which was incorrectly categorized as a command with incapsule data.

Also, add a code comment of what we are trying to do here.

Signed-off-by: Sagi Grimberg <sagi@grimberg.me>
Reviewed-by: Keith Busch <kbusch@kernel.org>
Signed-off-by: Christoph Hellwig <hch@lst.de>