mm/gup: Use huge_ptep_get() in gup_hugepte()

gup_hugepte() reads hugepage table entries, it can't read
them directly, huge_ptep_get() must be used.

Fixes: 9e343b46 ("READ_ONCE: Enforce atomicity for {READ,WRITE}_ONCE() memory accesses")
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Acked-by: Will Deacon <will@kernel.org>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/ffc3714334c3bfaca6f13788ad039e8759ae413f.1592225558.git.christophe.leroy@csgroup.eu

mm/gup.c

@@ -2425,7 +2425,7 @@ static int gup_hugepte(pte_t *ptep, unsigned long sz, unsigned long addr,
 	if (pte_end < end)
 		end = pte_end;
 
-	pte = READ_ONCE(*ptep);
+	pte = huge_ptep_get(ptep);
 
 	if (!pte_access_permitted(pte, flags & FOLL_WRITE))
 		return 0;