xen: Fix schedule()'s grabbing of the schedule lock

xen: Fix schedule()'s grabbing of the schedule lock

Because the location of the lock can change between the time you read
it and the time you grab it, the per-cpu schedule locks need to check
after lock acquisition that the lock location hasn't changed, and
release and re-try if so.  This change was effected throughout the
source code, but one very important place was apparently missed: in
schedule() itself.

Signed-off-by: George Dunlap <george.dunlap@eu.citrix.com>
Committed-by: Keir Fraser <keir@xen.org>