Pull Xen balloon driver bug-fixes from Stefano Stabellini:
- fix a preemption bug in xen/balloon.c;
- remove an harmful BUG_ON in xen/balloon.c that can trigger in
non-erroneous situations.
* tag 'stable/for-linus-3.12-rc0-tag-three' of git://git.kernel.org/pub/scm/linux/kernel/git/xen/tip:
xen/balloon: remove BUG_ON in increase_reservation
xen/balloon: ensure preemption is disabled when using a scratch page