Btrfs: Add backing store, memory management

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

diff --cc fs/btrfs/Makefile
Simple merge

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

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

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

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

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

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

diff --cc fs/btrfs/radix-tree.h
Simple merge