bcache: try to set b->parent properly

bcache_flash_dev.ktest would reliably crash with 8k and 16k bucket size
before; now it passes.

Change-Id: Ib542232235e39298c3a7548fe52b645cabb823d1

diff --cc drivers/md/bcache/btree.c
Simple merge

diff --cc drivers/md/bcache/btree.h
Simple merge

diff --cc drivers/md/bcache/super.c
Simple merge