dynticks: avoid flow_cache_flush() interrupting every core

Previously, if you did an "ifconfig down" or similar on one core, and
the kernel had CONFIG_XFRM enabled, every core would be interrupted to
check its percpu flow list for items that could be garbage collected.

With this change, we generate a mask of cores that actually have any
percpu items, and only interrupt those cores.  When we are trying to
isolate a set of cpus from interrupts, this is important to do.

Signed-off-by: Chris Metcalf <cmetcalf@tilera.com>
Signed-off-by: David S. Miller <davem@davemloft.net>