FROMLIST: mm: implement speculative handling in __do_fault()

In the speculative case, call the vm_ops->fault() method from within
an rcu read locked section, and verify the mmap sequence lock at the
start of the section. A match guarantees that the original vma is still
valid at that time, and that the associated vma->vm_file stays valid
while the vm_ops->fault() method is running.

Note that this implies that speculative faults can not sleep within
the vm_ops->fault method. We will only attempt to fetch existing pages
from the page cache during speculative faults; any miss (or prefetch)
will be handled by falling back to non-speculative fault handling.

The speculative handling case also does not preallocate page tables,
as it is always called with a pre-existing page table.

Signed-off-by: Michel Lespinasse <michel@lespinasse.org>
Link: https://lore.kernel.org/all/20210407014502.24091-25-michel@lespinasse.org/


Bug: 161210518
Signed-off-by: Suren Baghdasaryan <surenb@google.com>
Change-Id: I995ba94d8e96014ef83ac93fe5a4669afcde34b9