nvmet-tcp: remove boilerplate code

Simplify the nvmet_tcp_handle_h2c_data_pdu() function by removing
boilerplate code.

Signed-off-by: Maurizio Lombardi <mlombard@redhat.com>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Keith Busch <kbusch@kernel.org>