BACKPORT: FROMLIST: mm: implement speculative handling in filemap_fault()

Extend filemap_fault() to handle speculative faults.

In the speculative case, we will only be fishing existing pages out of
the page cache. The logic we use mirrors what is done in the
non-speculative case, assuming that pages are found in the page cache,
are up to date and not already locked, and that readahead is not
necessary at this time. In all other cases, the fault is aborted to be
handled non-speculatively.

Signed-off-by: Michel Lespinasse <michel@lespinasse.org>
Link: https://lore.kernel.org/all/20210407014502.24091-26-michel@lespinasse.org/



Conflicts:
    mm/filemap.c

1. Added back file_ra_state variable used by SPF path.
2. Updated comment for filemap_fault to reflect SPF locking rules.

Bug: 161210518
Signed-off-by: Suren Baghdasaryan <surenb@google.com>
Change-Id: I82eba7fcfc81876245c2e65bc5ae3d33ddfcc368
Git-commit: 59d4d125b7d0108b54860ea8584679d514ef07b0
Git-repo: https://android.googlesource.com/kernel/common/


[quic_c_spathi@quicinc.com: resolve trivial merge conflicts]
Signed-off-by: Srinivasarao Pathipati <quic_c_spathi@quicinc.com>