[SPARC64]: Wrap SMP IPIs with irq_enter()/irq_exit().

Otherwise all sorts of bad things can happen, including
spurious softlockup reports.

Other platforms have this same bug, in one form or
another, just don't see the issue because they
don't sleep as long as sparc64 can in NOHZ.

Thanks to some brilliant debugging by Peter Zijlstra.

Signed-off-by: David S. Miller <davem@davemloft.net>