return an ERR_PTR from __filemap_get_folio v2

[Christoph Hellwig]

On Tue, Mar 28, 2023 at 04:04:33PM -0700, Andrew Morton wrote:
> > Note that the shmem patches in here are non-trivial and need some
> > careful review and testing.
> 
> How are we going with the review and testing.  I assume that
> we're now OK on the runtime testing front, but do you feel that
> review has been adequate?

Yes, I think we're fine, mostly due to Hugh.  I'm a little sad about
the simplification / descoping from him, but at least we get the main
objective done.  Maybe at some point we can do another pass at
cleaning up the shmem page finding/reading mess.