sync.
VIPT/PIPT caches need valid VA-PA mapping in PTE for a cache operation to
succeed (unlike VIVT). Prior to this fix pmap was using l2pte_valid() for that
check, but this is not sufficient as the function merely checks if a PTE
exists (there can be existing but _invalid_ entries in the table).
A new pmap_has_valid_mapping() routine is introduced to do this job right by
checking proper PTE flags.
Among other potential problems this cures coherency issues with L2 caches on
MV-78100.
Submitted by: Grzegorz Bernacki, Piotr Ziecik
Reviewed, tested by: marcel
Obtained from: Semihalf
MFC after: 1 week
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