This is the number of bytes to be moved in the data transfer
associated with the command. The direction of the transfer is indicated
by 'dxfer_direction'. If 'dxfer_len' is zero then no data transfer
takes place.
[1]
If iovec_count is non-zero then 'dxfer_len' should be equal to the sum
of iov_len lengths. If not, the minimum of the two is the transfer length.
The type of dxfer_len is unsigned int.