FROMLIST: mm: implement and enable speculative fault handling in handle_pte_fault()

In handle_pte_fault(), allow speculative execution to proceed.

Use pte_spinlock() to validate the mmap sequence count when locking
the page table.

If speculative execution proceeds through do_wp_page(), ensure that we
end up in the wp_page_reuse() or wp_page_copy() paths, rather than
wp_pfn_shared() or wp_page_shared() (both unreachable as we only
handle anon vmas so far) or handle_userfault() (needs an explicit
abort to handle non-speculatively).

Signed-off-by: Michel Lespinasse <michel@lespinasse.org>
Link: https://lore.kernel.org/all/20220128131006.67712-28-michel@lespinasse.org/


Bug: 161210518
Signed-off-by: Suren Baghdasaryan <surenb@google.com>
Change-Id: Ia45d095ec7b8e23f1c5d68b7a7f572a3f6f6df97