xen: sched: improve checking soft-affinity

xen: sched: improve checking soft-affinity

The function has_soft_affinity() determines whether the soft-affinity
of a vcpu will have any effect -- that is, whether the affinity will
have any difference, scheduling-wise, from an empty soft-affinity
mask.

Such function takes a custom cpumask as its third parameter for better
flexibility; but that mask is different from the vCPU's hard-affinity
only in one case. Getting rid of that parameter not only simplifies
the function, but enables optimizing the soft affinity check.

It's mostly mechanical, with the exception of
sched_credit.c:_cshed_cpu_pick(), which was the one case where we
passed in something other than the existing hard-affinity.

Signed-off-by: Dario Faggioli <dfaggioli@suse.com>
Reviewed-by: George Dunlap <george.dunlap@citrix.com>