Btrfs: move over to use ->update_time

Btrfs had been doing it's own file_update_time so we could catch ENOSPC
properly, so just update our btrfs_update_time to work with the new stuff and
then we'll be fancy later.  Thanks,

Signed-off-by: Josef Bacik <josef@redhat.com>

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

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

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