xen: release lock occasionally during ballooning

When dom0 is being ballooned balloon_process() will hold the balloon
mutex until it is finished. This will block e.g. creation of new
domains as the device backends for the new domain need some
autoballooned pages for the ring buffers.

Avoid this by releasing the balloon mutex from time to time during
ballooning. Adjust the comment above balloon_process() regarding
multiple instances of balloon_process().

Instead of open coding it, just use cond_resched().

Signed-off-by: Juergen Gross <jgross@suse.com>
Signed-off-by: David Vrabel <david.vrabel@citrix.com>