Btrfs: A few updates for 2.6.18 and versions older than 2.6.25

This includes fixing a missing spinlock init call that caused oops on mount
for most kernels other than 2.6.25.

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

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

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

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