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.

Fixes: 6df7f764

 ("bpf, sockmap: Wake up polling after data copy")
Reported-by:  <syzbot+fd7b34375c1c8ce29c93@syzkaller.appspotmail.com>
Signed-off-by: Shigeru Yoshida <syoshida@redhat.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Tested-by:  <syzbot+fd7b34375c1c8ce29c93@syzkaller.appspotmail.com>
Acked-by: John Fastabend <john.fastabend@gmail.com>
Closes: https://syzkaller.appspot.com/bug?extid=fd7b34375c1c8ce29c93 [1]
Link: https://lore.kernel.org/bpf/20240218150933.6004-1-syoshida@redhat.com