xen: sched: Credit2: avoid looping too much (over runqueues) during load balancing

xen: sched: Credit2: avoid looping too much (over runqueues) during load balancing

For doing load balancing between runqueues, we check the load of each
runqueue, select the one more "distant" than our own load, and then take
the proper runq lock and attempt vcpu migrations.

If we fail to take such lock, we try again, and the idea was to give up
and bail if, during the checking phase, we can't take the lock of any
runqueue (check the comment near to the 'goto retry;', in the middle of
balance_load())

However, the variable that controls the "give up and bail" part, is not
reset upon retries. Therefore, provided we did manage to check the load of
at least one runqueue during the first pass, if we can't get any runq lock,
we don't bail, but we try again taking the lock of that same runqueue
(and that may even more than once).

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