ARM: fix rcu stalls on SMP platforms

commit 7deabca0acfe02b8e18f59a4c95676012f49a304 upstream.

We can stall RCU processing on SMP platforms if a CPU sits in its idle
loop for a long time.  This happens because we don't call irq_enter()
and irq_exit() around generic_smp_call_function_interrupt() and
friends.  Add the necessary calls, and remove the one from within
ipi_timer(), so that they're all in a common place.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>