Data digest calculation iterates over command mapped iovec. However
since commit bac04454 we unmap the iovec before we handle the data
digest, and since commit 69b85e1f we clear nr_mapped when we unmap
the iov.

Instead of open-coding the command iov traversal, simply call
crypto_ahash_digest with the command sg that is already allocated (we
already do that for the send path). Rename nvmet_tcp_send_ddgst to
nvmet_tcp_calc_ddgst and call it from send and recv paths.

Fixes: 69b85e1f ("nvmet-tcp: add an helper to free the cmd buffers")
Fixes: bac04454 ("nvmet-tcp: fix kmap leak when data digest in use")
Signed-off-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Christoph Hellwig <hch@lst.de>