Revise pmap_enter()'s handling of mapping updates that change the

PTE's PG_M and PG_RW bits but not the physical page frame.  First,
only perform vm_page_dirty() on a managed vm_page when the PG_M bit is
being cleared.  If the updated PTE continues to have PG_M set, then
there is no requirement to perform vm_page_dirty().  Second, flush the
mapping from the TLB when PG_M alone is cleared, not just when PG_M
and PG_RW are cleared.  Otherwise, a stale TLB entry may stop PG_M
from being set again on the next store to the virtual page.  However,
since the vm_page's dirty field already shows the physical page as
being dirty, no actual harm comes from the PG_M bit not being set.
Nonetheless, it is potentially confusing to someone expecting to see
the PTE change after a store to the virtual page.

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