archive/tar: reapply Header.Size to regFileReader after merging
The use of PAX headers can modify the overall file size, thus the
formerly created regFileReader may be stale.
The relevant PAX specification for this behavior is:
<<<
Any fields in the preceding optional extended header shall override
the associated fields in this header block for this file.
>>>
Where "optional extended header" refers to the preceding PAX header.
Where "this header block" refers to the subsequent USTAR header.