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 <[e-mail]>
Acked-by: Matt Fleming <[e-mail]>
Signed-off-by: Kees Cook <[e-mail]>

drivers/firmware/efi/efi-pstore.c

···
 
 static __exit void efivars_pstore_exit(void)
 {
+	if (!efi_pstore_info.bufsize)
+		return;
+
+	pstore_unregister(&efi_pstore_info);
+	kfree(efi_pstore_info.buf);
+	efi_pstore_info.buf = NULL;
+	efi_pstore_info.bufsize = 0;
 }
 
 module_init(efivars_pstore_init);