When the mapping betwween the Linux notion of CPU ID to the hypervisor's
notion of CPU ID is not initialized, IPI must fall back to the
non-enlightened path.

The recent merge of upstream changes into the hyperv branch resolved a
merge conflict wronly by returning success in that case, which results in
the IPI not being sent at all. Fix it up.

Fixes: 8f63e923 ("Merge branch 'x86/urgent' into x86/hyperv")
Reported-by: NMichael Kelley <mikelley@microsoft.com>
Signed-off-by: NK. Y. Srinivasan <kys@microsoft.com>
Signed-off-by: NThomas Gleixner <tglx@linutronix.de>
Cc: gregkh@linuxfoundation.org
Cc: devel@linuxdriverproject.org
Cc: olaf@aepfle.de
Cc: apw@canonical.com
Cc: jasowang@redhat.com
Cc: hpa@zytor.com
Cc: sthemmin@microsoft.com
Cc: Michael.H.Kelley@microsoft.com
Cc: vkuznets@redhat.com
Link: https://lkml.kernel.org/r/20180720035009.3995-1-kys@linuxonhyperv.com