Btrfs: only update disk_i_size as we remove extents

This fixes a problem where if we fail a truncate we will leave the i_size set
where we wanted to truncate to instead of where we were able to truncate to.
Fix this by making btrfs_truncate_inode_items do the disk_i_size update as it
removes extents, that way it will always be consistent with where its extents
are.  Then if the truncate fails at all we can update the in-ram i_size with
what we have on disk and delete the orphan item.  Thanks,

Signed-off-by: Josef Bacik <jbacik@fusionio.com>
Signed-off-by: Chris Mason <chris.mason@fusionio.com>