BACKPORT: FROMLIST: mm: implement speculative fault handling in finish_fault()

In the speculative case, we want to avoid direct pmd checks (which
would require some extra synchronization to be safe), and rely on
pte_map_lock which will both lock the page table and verify that the
pmd has not changed from its initial value.

Signed-off-by: Michel Lespinasse <michel@lespinasse.org>
Link: https://lore.kernel.org/all/20210407014502.24091-27-michel@lespinasse.org/



Conflicts:
    mm/memory.c

1. Merge conflict due to new vmf->prealloc_pte usage in finish_fault.

Bug: 161210518
Signed-off-by: Suren Baghdasaryan <surenb@google.com>
Change-Id: If6046592083eaf12caf5c51c3fbb287a4dfa1ace