genirq/cpuhotplug: Add sanity check for effective affinity mask

genirq/cpuhotplug: Add sanity check for effective affinity mask

BugLink: http://bugs.launchpad.net/bugs/1724669
commit 60b09c51bb4fb46e2331fdbb39f91520f31d35f7 upstream.

The effective affinity mask handling has no safety net when the mask is not
updated by the interrupt chip or the mask contains offline CPUs.

If that happens the CPU unplug code fails to migrate interrupts.

Add sanity checks and emit a warning when the mask contains only offline
CPUs.

Fixes: 415fcf1a2293 ("genirq/cpuhotplug: Use effective affinity mask")
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Marc Zyngier <marc.zyngier@arm.com>
Cc: Christoph Hellwig <hch@lst.de>
Link: http://lkml.kernel.org/r/alpine.DEB.2.20.1710042208400.2406@nanos
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Seth Forshee <seth.forshee@canonical.com>