mm/readahead: Do not allow order-1 folio

The THP machinery does not support order-1 folios because it requires
meta data spanning the first 3 `struct page`s. So order-2 is the
smallest large folio that we can safely create.

There was a theoretical bug whereby if ra->size was 2 or 3 pages (due to
the device-specific bdi->ra_pages being set that way), we could end up
with order = 1. Fix this by unconditionally checking if the preferred
order is 1 and if so, set it to 0. Previously this was done in a few
specific places, but with this refactoring it is done just once,
unconditionally, at the end of the calculation.

This is a theoretical bug found during review of the code; I have no
evidence to suggest this manifests in the real world (I expect all
device-specific ra_pages values are much bigger than 3).

Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>