genirq: fix handle_nested_irq for lazy disable

When lazy disabling is implemented and an interrupt is disabled the
genirq code ends up marking it as IRQ_DISABLED in the descriptor.
The interrupt stays enabled in the controller.  If the interrupt
fires after disabling, the flow handlers namely handle_level_irq and
handle_edge_irq mask the interrupt in the controller.

This is not the case with handle_nested_irq. The interrupt stays enabled in
the controller and if it were a level interrupt it keeps firing only to be
ignored by handle_nested_irq.

Update handle_nested_irq to mask such an interrupt.

Change-Id: I9412e0bbf796c12a13b29802fe95fd544592bff5
CRs-Fixed: 300931
Signed-off-by: Abhijeet Dharmapurikar <adharmap@codeaurora.org>
Signed-off-by: Stepan Moskovchenko <stepanm@codeaurora.org>