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.

Fixes #15573
Fixes #15564

Change-Id: I83b1c3f05a9ca2d3be38647425ad21a9fe450ee2
Reviewed-on: https://go-review.googlesource.com/28418Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>