x86/apic: Remove noisy zero-mask warning from default_send_IPI_mask_logical()

x86/apic: Remove noisy zero-mask warning from default_send_IPI_mask_logical()

Since circa 3.5, we've had dozens of reports of people hitting
this warning. Forwarded reports have been met with silence, so
just remove the warning if no-one cares.

Example reports:

  https://bugzilla.redhat.com/show_bug.cgi?id=797687
  https://bugzilla.redhat.com/show_bug.cgi?id=867174
  https://bugzilla.redhat.com/show_bug.cgi?id=894865

Signed-off-by: Dave Jones <davej@redhat.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Link: http://lkml.kernel.org/r/20130118175847.GA27662@redhat.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>