xen/balloon: set a mapping for ballooned out pages

xen/balloon: set a mapping for ballooned out pages

Currently ballooned out pages are mapped to 0 and have INVALID_P2M_ENTRY
in the p2m. These ballooned out pages are used to map foreign grants
by gntdev and blkback (see alloc_xenballooned_pages).

Allocate a page per cpu and map all the ballooned out pages to the
corresponding mfn. Set the p2m accordingly. This way reading from a
ballooned out page won't cause a kernel crash (see
http://lists.xen.org/archives/html/xen-devel/2012-12/msg01154.html).

Signed-off-by: Stefano Stabellini <[email protected]>
Reviewed-by: David Vrabel <[email protected]>
CC: [email protected]
CC: [email protected]
Signed-off-by: Konrad Rzeszutek Wilk <[email protected]>