rshim boot interface copy hang with larger buffers

[thillux]

When using dd to copy a bootstream to the rshim, I noticed that larger buffer sizes (e.g. 1MB) never finish copying. The last buffer always does not finish copying.

Example:
dd if=installer.bf of=/dev/rshim0/boot bs=1M conv=sync status=progress

When terminating dd with SIGINT, it shows the following:
435159040 bytes (435 MB, 415 MiB) copied, 80 s, 5.4 MB/s
^C
441+1 records in
441+0 records out
462422016 bytes (462 MB, 441 MiB) copied, 170.944 s, 2.7 MB/s

My bfb file was 442 MB large, so the last buffer is missing/never finished copying.