The radix tree retry logic got a bit of an overhaul and subsequently
broke the virtual IRQ subgroup build. Simply switch over to
radix_tree_deref_retry() as per the filemap changes, which the virq
lookup logic was modelled after in the first place.
Signed-off-by: NPaul Mundt <lethal@linux-sh.org>

Signed-off-by: NPaul Mundt <lethal@linux-sh.org>

This splits up the sh intc core in to something more vaguely resembling
a subsystem. Most of the functionality was alread fairly well
compartmentalized, and there were only a handful of interdependencies
that needed to be resolved in the process.

This also serves as future-proofing for the genirq and sparseirq rework,
which will make some of the split out functionality wholly generic,
allowing things to be killed off in place with minimal migration pain.
Signed-off-by: NPaul Mundt <lethal@linux-sh.org>