alpha: Replace smp_read_barrier_depends() usage with smp_[r]mb()

In preparation for removing smp_read_barrier_depends() altogether,
move the Alpha code over to using smp_rmb() and smp_mb() directly.

Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Will Deacon <will@kernel.org>
diff --git a/mm/memory.c b/mm/memory.c
index 87ec87c..e1f2c73 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -437,7 +437,7 @@ int __pte_alloc(struct mm_struct *mm, pmd_t *pmd)
 	 * of a chain of data-dependent loads, meaning most CPUs (alpha
 	 * being the notable exception) will already guarantee loads are
 	 * seen in-order. See the alpha page table accessors for the
-	 * smp_read_barrier_depends() barriers in page table walking code.
+	 * smp_rmb() barriers in page table walking code.
 	 */
 	smp_wmb(); /* Could be smp_wmb__xxx(before|after)_spin_lock */