From: NeilBrown <neilb@cse.unsw.edu.au>

next_r1 is never used, so it can just go.

read_bio isn't needed as we can easily use one of the pointers in the
write_bios array - write_bios[->read_disk].  So rename "write_bios" to "bios"
and store the pointer to the read bio in there.