bcache: Take data offset from the bdev superblock.

Add a new superblock version, and consolidate related defines.

Signed-off-by: Gabriel de Perthuis <g2p.code+bcache@gmail.com>
Signed-off-by: Kent Overstreet <koverstreet@google.com>

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

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

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