include/linux: Remove smp_read_barrier_depends() from comments

smp_read_barrier_depends() doesn't exist any more, so reword the two
comments that mention it to refer to "dependency ordering" instead.

Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Will Deacon <will@kernel.org>