tar: fix memory leaks when processing symlinks or parsing pax headers (#2341)

tar: fix memory leaks when processing symlinks or parsing pax headers (#2341)

Fix memory leaks introduced by #2127:

* `struct tar` member `entry_linkpath` was moved at the same time as
other members were removed, but its cleanup was accidentally removed
with the others.

* `header_pax_extension` local variable `attr_name` was not cleaned up.

Resolves #2336

(cherry picked from commit 7c3980367e79c3f89e9ba595bbe67b3983b26215)

Co-authored-by: Brad King <brad.king@kitware.com>