Impact: fix bug with irq-descriptor moving when logical flat

Rusty observed:

> The effect of setting desc->affinity (ie. from userspace via sysfs) has varied
> over time.  In 2.6.27, the 32-bit code anded the value with cpu_online_map,
> and both 32 and 64-bit did that anding whenever a cpu was unplugged.
>
> 2.6.29 consolidated this into one routine (and fixed hotplug) but introduced
> another variation: anding the affinity with cfg->domain.  Is this right, or
> should we just set it to what the user said?  Or as now, indicate that we're
> restricting it.

Eric pointed out that desc->affinity should be what the user requested,
if it is at all possible to honor the user space request.

This bug got introduced by commit 22f65d31 "x86: Update io_apic.c to use
new cpumask API".

Fix it by moving the masking to before the descriptor moving ...
Reported-by: Rusty Russell <rusty@rustcorp.com.au>
Reported-by: Eric W. Biederman <ebiederm@xmission.com>
LKML-Reference: <49C94134.4000408@kernel.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>