bpf, sockmap: Fix NULL pointer dereference in sk_psock_verdict_data_ready()

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

bpf, sockmap: Fix NULL pointer dereference in sk_psock_verdict_data_ready()

syzbot reported the following NULL pointer dereference issue [1]:

BUG: kernel NULL pointer dereference, address: 0000000000000000

[...]

RIP: 0010:0x0

[...]

Call Trace:

<TASK>

sk_psock_verdict_data_ready+0x232/0x340 net/core/skmsg.c:1230

unix_stream_sendmsg+0x9b4/0x1230 net/unix/af_unix.c:2293

sock_sendmsg_nosec net/socket.c:730 [inline]

__sock_sendmsg+0x221/0x270 net/socket.c:745

____sys_sendmsg+0x525/0x7d0 net/socket.c:2584

___sys_sendmsg net/socket.c:2638 [inline]

__sys_sendmsg+0x2b0/0x3a0 net/socket.c:2667

do_syscall_64+0xf9/0x240

entry_SYSCALL_64_after_hwframe+0x6f/0x77

If sk_psock_verdict_data_ready() and sk_psock_stop_verdict() are called

concurrently, psock->saved_data_ready can be NULL, causing the above issue.

This patch fixes this issue by calling the appropriate data ready function

using the sk_psock_data_ready() helper and protecting it from concurrency

with sk->sk_callback_lock.