ptr_ring: READ/WRITE_ONCE for __ptr_ring_empty

Lockless __ptr_ring_empty requires that consumer head is read and
written at once, atomically. Annotate accordingly to make sure compiler
does it correctly.  Switch locked callers to __ptr_ring_peek which does
not support the lockless operation.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>