mm: Fix pmd_read_atomic()

AFAICT there's no reason to do anything different than what we do for
PTEs. Make it so (also affects SH).

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20221022114424.711181252%40infradead.org