Btrfs: Add back the online defragging code

Signed-off-by: Chris Mason <chris.mason@oracle.com>

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

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

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

diff --cc fs/btrfs/extent-tree.c
Simple merge

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

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

diff --cc fs/btrfs/tree-defrag.c
Simple merge