nvmet-tcp: remove nvmet_tcp_finish_cmd

There is only a single call-site of nvmet_tcp_finish_cmd(), this
becomes redundant. Remove nvmet_tcp_finish_cmd() and use the original
function body instead.

Suggested-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: zhenwei pi <pizhenwei@bytedance.com>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Christoph Hellwig <hch@lst.de>