FROMLIST: mm: implement speculative handling in wp_page_copy()

Change wp_page_copy() to handle the speculative case. This involves
aborting speculative faults if they have to allocate an anon_vma,
read-locking the mmu_notifier_lock to avoid races with
mmu_notifier_register(), and using pte_map_lock() instead of
pte_offset_map_lock() to complete the page fault.

Also change call sites to clear vmf->pte after unmapping the page table,
in order to satisfy pte_map_lock()'s preconditions.

Signed-off-by: Michel Lespinasse <michel@lespinasse.org>
Link: https://lore.kernel.org/all/20220128131006.67712-27-michel@lespinasse.org/


Bug: 161210518
Signed-off-by: Suren Baghdasaryan <surenb@google.com>
Change-Id: Icd2188e9facf5a7fea42000a2808bcda1ad6f0fc