kthread: Fix return value on kzalloc() failure in kthread_affine_preferred()

kthread_affine_preferred() incorrectly returns 0 instead of -ENOMEM
when kzalloc() fails. Return 'ret' to ensure the correct error code is
propagated.

Fixes: 4d13f430 ("kthread: Implement preferred affinity")
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202501301528.t0cZVbnq-lkp@intel.com/


Signed-off-by: Yu-Chun Lin <eleanor15x@gmail.com>
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>