Remove offset field from struct btrfs_extent_ref

Remove offset field from struct btrfs_extent_ref

The offset field in struct btrfs_extent_ref records the position
inside file that file extent is referenced by. In the new back
reference system, tree leaves holding reference to file extent
are recorded explicitly. We can quickly scan these tree leaves, so the
offset field is not required.

This patch also makes the back reference system check the objectid
when extents are being deleted

Signed-off-by: Yan Zheng <zheng.yan@oracle.com>