xen/m2p: use GNTTABOP_unmap_and_replace to reinstate the original mapping

xen/m2p: use GNTTABOP_unmap_and_replace to reinstate the original mapping

GNTTABOP_unmap_grant_ref unmaps a grant and replaces it with a 0
mapping instead of reinstating the original mapping.
Doing so separately would be racy.

To unmap a grant and reinstate the original mapping atomically we use
GNTTABOP_unmap_and_replace.
GNTTABOP_unmap_and_replace doesn't work with GNTMAP_contains_pte, so
don't use it for kmaps.  GNTTABOP_unmap_and_replace zeroes the mapping
passed in new_addr so we have to reinstate it, however that is a
per-cpu mapping only used for balloon scratch pages, so we can be sure that
it's not going to be accessed while the mapping is not valid.

Signed-off-by: Stefano Stabellini <[email protected]>
Reviewed-by: David Vrabel <[email protected]>
Acked-by: Konrad Rzeszutek Wilk <[email protected]>
CC: [email protected]
CC: [email protected]
[v1: Konrad fixed up the conflicts]
Conflicts:
	arch/x86/xen/p2m.c