(1) The reads are always done from the first n/2 disks.
(2) Each write is done twice, to the "data" disk (in the first half) and
the "mirror" disk (in the second half).
ccdbuffer() now takes an extra argument (struct ccdbuf **) and stores
the pointer to ccdbuf in there. In case of a mirrored write, it
allocates and stores two pointers. The "residual" is also doubled
for mirrored writes so that ccdiodone() can correctly tell when all
the writes are done.
