[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 <email@example.com>