afs: fix the afs_dir_get_folio return value

Keep returning NULL on failure instead of letting an ERR_PTR escape to
callers that don't expect it.

Link: https://lkml.kernel.org/r/20230503154526.1223095-2-hch@lst.de
Fixes: 66dabbb6

 ("mm: return an ERR_PTR from __filemap_get_folio")
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reported-by: Jan Kara <jack@suse.cz>
Reviewed-by: Jan Kara <jack@suse.cz>
Reviewed-by: David Howells <dhowells@redhat.com>
Tested-by: David Howells <dhowells@redhat.com>
Cc: Marc Dionne <marc.dionne@auristor.com>
Cc: Matthew Wilcox <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>