nvmet-tcp: fix data digest pointer calculation

exp_ddgst is of type __le32, &cmd->exp_ddgst + cmd->offset increases
&cmd->exp_ddgst by 4 * cmd->offset, fix this by type casting
&cmd->exp_ddgst to u8 *.

Fixes: 872d26a3 ("nvmet-tcp: add NVMe over TCP target driver")
Signed-off-by: Varun Prakash <varun@chelsio.com>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Christoph Hellwig <hch@lst.de>