xfrm: espintcp: reorder operations within espintcp_init_sk

When enabled on a socket, espintcp sets the socket callbacks to its
own before it has finished setting up its context and published it as
icsk_ulp_data. Any one of those callbacks that gets called before will
dereference a NULL icsk_ulp_data.

Fix this by reording the operations, and add memory barriers to ensure
callers will have a struct espintcp_ctx available. Add a few
WRITE_ONCE/READ_ONCE too while we're there.

Fixes: e27cca96cd68 ("xfrm: add espintcp (RFC 8229)")
Cc: stable@vger.kernel.org
Reported-by: Yuan Tan <yuantan098@gmail.com>
Reported-by: Yifan Wu <yifanwucs@gmail.com>
Reported-by: Juefei Pu <tomapufckgml@gmail.com>
Reported-by: Xin Liu <bird@lzu.edu.cn>
Reported-by: Peihan Liu <ronbogo@outlook.com>
Reported-by: Yilin Zhu <zylzyl2333@gmail.com>
Reported-by: Ren Wei <n05ec@lzu.edu.cn>
Reported-by: Eulgyu Kim <eulgyukim@snu.ac.kr>
Reported-by: Jaeyoung Chung <jjy600901@snu.ac.kr>
Link: https://lore.kernel.org/all/c30b645074a1b379e0f7fe297f917c66137d9964.1778464688.git.zylzyl2333@gmail.com/
Link: https://lore.kernel.org/all/20260819155349.3555804-1-jjy600901@snu.ac.kr
Signed-off-by: Sabrina Dubroca <sd@queasysnail.net>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>