Btrfs: add GPLv2

fs/btrfs/COPYING

fs/btrfs/TODO

fs/btrfs/bit-radix.c

fs/btrfs/bit-radix.h

fs/btrfs/btrfs_inode.h

fs/btrfs/ctree.c

fs/btrfs/ctree.h

fs/btrfs/dir-item.c

fs/btrfs/disk-io.c

fs/btrfs/disk-io.h

fs/btrfs/extent-tree.c

fs/btrfs/file-item.c

fs/btrfs/file.c

fs/btrfs/hash.c

fs/btrfs/hash.h

fs/btrfs/inode-item.c

fs/btrfs/inode-map.c

fs/btrfs/inode.c

fs/btrfs/ioctl.h

fs/btrfs/print-tree.c