nvmet-tcp: fix maxh2cdata icresp parameter

MAXH2CDATA is not zero based. Also no reason to limit ourselves to
1M transfers as we can do more easily. Make this an arbitrary limit
of 16M.

Reported-by: Wenhua Liu <liuw@vmware.com>
Cc: stable@vger.kernel.org # v5.0+
Signed-off-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Keith Busch <kbusch@kernel.org>