IOMMU: clear "don't flush" override on error paths

IOMMU: clear "don't flush" override on error paths

Both xenmem_add_to_physmap() and iommu_populate_page_table() each have
an error path that fails to clear that flag, thus suppressing further
flushes on the respective pCPU.

In iommu_populate_page_table() also slightly re-arrange code to avoid
the false impression of the flag in question being guarded by a
domain's page_alloc_lock.

This is CVE-2013-6400 / XSA-80.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>