irqchip/mips-gic: Don't overrun pcpu_masks array

Commit 2a078705 ("irqchip/mips-gic: Use gic_vpes instead of
NR_CPUS") & commit 78930f09 ("irqchip/mips-gic: Clear percpu_masks
correctly when mapping") both introduce code which accesses gic_vpes
entries in the pcpu_masks array. However, this array has length NR_CPUS.
If NR_CPUS is less than gic_vpes (ie. the kernel supports use of less
CPUs than are present in the system) then we overrun the array, clobber
some other data & generally die pretty promptly.

Most notably this affects uniprocessor kernels running on any multicore
or multithreaded Malta with a GIC (ie. the vast majority of real Malta
boards).

Fix this by only accessing up to min(gic_vpes, NR_CPUS) entries in the
pcpu_masks array, preventing the array overrun.

Fixes: 2a078705 ("irqchip/mips-gic: Use gic_vpes instead of NR_CPUS")
Fixes: 78930f09

 ("irqchip/mips-gic: Clear percpu_masks correctly when mapping")
Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Cc: linux-mips@linux-mips.org
Cc: Jason Cooper <jason@lakedaemon.net>
Cc: Marc Zyngier <marc.zyngier@arm.com>
Cc: Ralf Baechle <ralf@linux-mips.org>
Link: http://lkml.kernel.org/r/1461234714-9975-1-git-send-email-paul.burton@imgtec.com
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>