Btrfs: change core code of btrfs to support the device replace operations

This commit contains all the essential changes to the core code
of Btrfs for support of the device replace procedure.

Signed-off-by: Stefan Behrens <sbehrens@giantdisaster.de>
Signed-off-by: Chris Mason <chris.mason@fusionio.com>

diff --cc fs/btrfs/disk-io.c
Simple merge

diff --cc fs/btrfs/reada.c
Simple merge

diff --cc fs/btrfs/scrub.c
Simple merge

diff --cc fs/btrfs/super.c
Simple merge

diff --cc fs/btrfs/transaction.c
Simple merge

diff --cc fs/btrfs/volumes.c
Simple merge

diff --cc fs/btrfs/volumes.h
Simple merge