This patch fixes a corner case where the asoc out stream count may change
after wait_for_sndbuf.

When the main thread in the client starts a connection, if its out stream
count is set to N while the in stream count in the server is set to N - 2,
another thread in the client keeps sending the msgs with stream number
N - 1, and waits for sndbuf before processing INIT_ACK.

However, after processing INIT_ACK, the out stream count in the client is
shrunk to N - 2, the same to the in stream count in the server. The crash
occurs when the thread waiting for sndbuf is awake and sends the msg in a
non-existing stream(N - 1), the call trace is as below:

  KASAN: null-ptr-deref in range [0x0000000000000038-0x000000000000003f]
  Call Trace:
   <TASK>
   sctp_cmd_send_msg net/sctp/sm_sideeffect.c:1114 [inline]
   sctp_cmd_interpreter net/sctp/sm_sideeffect.c:1777 [inline]
   sctp_side_effects net/sctp/sm_sideeffect.c:1199 [inline]
   sctp_do_sm+0x197d/0x5310 net/sctp/sm_sideeffect.c:1170
   sctp_primitive_SEND+0x9f/0xc0 net/sctp/primitive.c:163
   sctp_sendmsg_to_asoc+0x10eb/0x1a30 net/sctp/socket.c:1868
   sctp_sendmsg+0x8d4/0x1d90 net/sctp/socket.c:2026
   inet_sendmsg+0x9d/0xe0 net/ipv4/af_inet.c:825
   sock_sendmsg_nosec net/socket.c:722 [inline]
   sock_sendmsg+0xde/0x190 net/socket.c:745

The fix is to add an unlikely check for the send stream number after the
thread wakes up from the wait_for_sndbuf.

Fixes: 5bbbbe32 ("sctp: introduce stream scheduler foundations")
Reported-by: syzbot+47c24ca20a2fa01f082e@syzkaller.appspotmail.com
Signed-off-by: Xin Long <lucien.xin@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>