kthread: Warn about failed allocations for the init kthread

Failed allocates are not expected when setting up the initial task and
it is not really possible to handle them either.  So I added a warning
to report if such an allocation failure ever happens.

Correct the sense of the warning so it warns when an allocation failure
happens not when the allocation succeeded.  Oops.

Reported-by: kernel test robot <oliver.sang@intel.com>
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Reported-by: Naresh Kamboju <naresh.kamboju@linaro.org>
Link: https://lkml.kernel.org/r/20211221231611.785b74cf@canb.auug.org.au
Link: https://lkml.kernel.org/r/CA+G9fYvLaR5CF777CKeWTO+qJFTN6vAvm95gtzN+7fw3Wi5hkA@mail.gmail.com
Link: https://lkml.kernel.org/r/20211216102956.GC10708@xsang-OptiPlex-9020
Fixes: 40966e31

 ("kthread: Ensure struct kthread is present for all kthreads")
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>