In preparation for removing smp_read_barrier_depends() altogether,
move the Alpha code over to using smp_rmb() and smp_mb() directly.
Acked-by: NPeter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: NPaul E. McKenney <paulmck@kernel.org>
Signed-off-by: NWill Deacon <will@kernel.org>