mips/gup: Replace ACCESS_ONCE with READ_ONCE

ACCESS_ONCE does not work reliably on non-scalar types. For
example gcc 4.6 and 4.7 might remove the volatile tag for such
accesses during the SRA (scalar replacement of aggregates) step
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=58145)

Change the gup code to replace ACCESS_ONCE with READ_ONCE.

Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
Acked-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

arch/mips/mm/gup.c

@@ -30,7 +30,7 @@ retry:
 
 	return pte;
 #else
-	return ACCESS_ONCE(*ptep);
+	return READ_ONCE(*ptep);
 #endif
 }