FROMLIST: mm: implement speculative handling in do_fault_around()

Call the vm_ops->map_pages method within an rcu read locked section.
In the speculative case, 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->map_pages() method is running.

Do not test vmf->pmd in the speculative case - we only speculate when
a page table already exists, and and this saves us from having to handle
synchronization around the vmf->pmd read.

Change xfs_filemap_map_pages() account for the fact that it can not
block anymore, as it is now running within an rcu read lock.

Signed-off-by: Michel Lespinasse <michel@lespinasse.org>
Link: https://lore.kernel.org/all/20210407014502.24091-28-michel@lespinasse.org/


Bug: 161210518
Signed-off-by: Suren Baghdasaryan <surenb@google.com>
Change-Id: Id771c1e6fa9b883595a48d4df63f448a05916eda