efi/efivars: Add missing kobject_put() in sysfs entry creation error path

The documentation provided by kobject_init_and_add() clearly spells out
the need to call kobject_put() on the kobject if an error is returned.
Add this missing call to the error path.

Cc: <stable@vger.kernel.org>
Reported-by: 亿一 <teroincn@gmail.com>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>