add_disk_randomness() is causing a context switch per disk request.  It
is scheduling process-context work one timer tick in the future for
every request.

But is has a buffer for this, so change it to not set up the
process-context work until that buffer is half full.

The patch reduces the context switch rate during a 20 megabyte/sec
write to scsi from 150/sec to 50/sec.