efi-pstore: implement efivars_pstore_exit()

The original efivars_pstore_exit() is empty. I
 1) add a bufsize check statement.
 2) call pstore_unregister as it is defined now.
 3) free the memory and set bufsize to 0.

Signed-off-by: Geliang Tang <geliangtang@163.com>
Acked-by: Matt Fleming <matt@codeblueprint.co.uk>
Signed-off-by: Kees Cook <keescook@chromium.org>