mm: More ptep_get() conversion

Commit c33c7948 ("mm: ptep_get() conversion") converted all
(non-arch) call sites to use ptep_get() instead of doing a direct
dereference of the pte. Full rationale can be found in that commit's
log.

Since then, three new call sites have snuck in, which directly
dereference the pte, so let's fix those up.

Unfortunately there is no reliable automated mechanism to catch these;
I'm relying on a combination of Coccinelle (which throws up a lot of
false positives) and some compiler magic to force a compiler error on
dereference (While this approach finds dereferences, it also yields a
non-booting kernel so can't be committed).

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