tar: fix memory leaks when processing symlinks or parsing pax headers (#2338)

tar: fix memory leaks when processing symlinks or parsing pax headers (#2338)

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