CVE-2023-53839

In the Linux kernel, the following vulnerability has been resolved:

dccp: fix data-race around dp->dccps_mss_cache

dccp_sendmsg() reads dp->dccps_mss_cache before locking the socket.
Same thing in do_dccp_getsockopt().

Add READ_ONCE()/WRITE_ONCE() annotations,
and change dccp_sendmsg() to check again dccps_mss_cache
after socket is locked.