mptcp: fix subflow bitfield misuse

The subflow status bitfield is protected by the subflow socket lock,
with one notable exception: the `close_event_done` bit is actually under
the msk socket lock protection.

As reported by Clashiko [1], flipping the latter bit may corrupt other
entries in the same bitfield.

Move the `close_event_done` outside the bitfield, in a 32bit hole.

Link: https://lore.kernel.org/178959153492.22033.1944011998010756543@kernel.org
Fixes: d82809b6c5 ("mptcp: avoid duplicated SUB_CLOSED events")
Cc: stable@vger.kernel.org
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Reviewed-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20261013-net-mptcp-misc-fixes-7-3-rc8-v1-2-932c9b182fc7@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

net/mptcp/protocol.h

@@ -583,14 +583,16 @@ struct mptcp_subflow_context {
 		closing : 1,	    /* must not pass rx data to msk anymore */
 		valid_csum_seen : 1,        /* at least one csum validated */
 		is_mptfo : 1,	    /* subflow is doing TFO */
-		close_event_done : 1,       /* has done the post-closed part */
 		mpc_drop : 1,	    /* the MPC option has been dropped in a rtx */
 		resetting : 1,	    /* subflow is resetting */
-		__unused : 8;
+		__unused : 9;
 	bool	data_avail;
 	bool	scheduled;
 	bool	pm_listener;	    /* a listener managed by the kernel PM? */
 	bool	fully_established;  /* path validated */
+	bool	close_event_done;   /* has done the post-closed part,
+				     * protected by msk socket lock
+				     */
 	u32	lent_mem_frag;
 	u32	remote_nonce;
 	u64	thmac;