Revert one of the changes from r349323. Specifically, undo the change
that replaced a pmap_invalidate_page() with a dsb(ishst) in
pmap_enter_quick_locked(). Even though this change is in principle
correct, I am seeing occasional, spurious bus errors that are only
reproducible without this pmap_invalidate_page(). (None of adding an
isb, "upgrading" the dsb to wait on loads as well as stores, or
disabling superpage mappings eliminates the bus errors.) Add an XXX
comment explaining why the pmap_invalidate_page() is being performed.

Discussed with: andrew, markj