nvmet-tcp: fix a crash in nvmet_req_complete()

in nvmet_tcp_handle_h2c_data_pdu(), if the host sends a data_offset
different from rbytes_done, the driver ends up calling nvmet_req_complete()
passing a status error.
The problem is that at this point cmd->req is not yet initialized,
the kernel will crash after dereferencing a NULL pointer.

Fix the bug by replacing the call to nvmet_req_complete() with
nvmet_tcp_fatal_error().

Fixes: 872d26a3

 ("nvmet-tcp: add NVMe over TCP target driver")
Reviewed-by: Keith Busch <kbsuch@kernel.org>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Maurizio Lombardi <mlombard@redhat.com>
Signed-off-by: Keith Busch <kbusch@kernel.org>