IOMMU/EPT: avoid double flushing in shared page table case

IOMMU/EPT: avoid double flushing in shared page table case

While the flush coalescing optimization has been helping the non-shared
case, it has actually lead to double flushes in the shared case (which
ought to be the more common one nowadays at least): Once from
*_set_entry() and a second time up the call tree from wherever the
overriding flag gets played with. In alignment with XSA-346 suppress
flushing in this case.

Similarly avoid excessive setting of IOMMU_FLUSHF_added on the batched
flushes: "idx" hasn't been added a new mapping for.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>