amt: use READ_ONCE() in amt module

There are some data races in the amt module.
amt->ready4, amt->ready6, and amt->status can be accessed concurrently
without locks.
So, it uses READ_ONCE() and WRITE_ONCE().

Fixes: cbc21dc1 ("amt: add data plane of amt interface")
Signed-off-by: Taehee Yoo <ap420073@gmail.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>