sctp: hold transport reference across connect wait

sctp_sendmsg_to_asoc() selects a transport before waiting for a new
interleaved association to become established. sctp_wait_for_connect()
drops the socket lock while sleeping.

While the lock is dropped, backlog processing can handle a COOKIE ACK and
an ASCONF DEL-IP that removes the selected transport. sctp_assoc_rm_peer()
then marks the transport dead and drops its association-owned reference.
The transport can be freed through RCU before the send path resumes.

After the wait, the stale pointer is assigned to newly created DATA or
I-DATA chunks and later dereferenced by sctp_outq_select_transport().

Hold a reference to the selected transport across
sctp_wait_for_connect(). After the socket lock is reacquired, abort the
send if the transport was marked dead. Callers without a selected
transport pass NULL and retain their existing behavior.

Cc: stable@vger.kernel.org
Fixes: 668c9beb9020 ("sctp: implement assign_number for sctp_stream_interleave")
Acked-by: Xin Long <lucien.xin@gmail.com>
Signed-off-by: Seung Min Shin <guncraft2000@naver.com>
Link: https://patch.msgid.link/20261011162750.1529631-1-guncraft2000@naver.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>