ksmbd: remove stale channels from all sessions on teardown

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

ksmbd: remove stale channels from all sessions on teardown

ksmbd_sessions_deregister() removes a connection's channels from other
sessions' channel lists only while conn->binding is still set:

if (conn->binding) {
hash_for_each_safe(sessions_table, ...)
ksmbd_chann_del(conn, sess);
}

conn->binding is a transient flag: it is cleared once a binding
SESSION_SETUP completes, and also by a subsequent non-binding
SESSION_SETUP on the same connection (a reauthentication on a bound
channel, or a new SessionId==0 setup). A connection that has bound a
channel into another session's ksmbd_chann_list and then clears
conn->binding leaves that channel behind when it disconnects: the
channel, whose chann->conn points at the now freed struct ksmbd_conn,
stays on the owner session's list.

When the owning connection later tears down, the second loop
dereferences the stale channel:

xa_for_each(&sess->ksmbd_chann_list, chann_id, chann)
if (chann->conn != conn)
ksmbd_conn_set_exiting(chann->conn); /* freed */

which is a use-after-free write into the freed ksmbd_conn (the same
stale channel is also walked by show_proc_session() through /proc). The
session is leaked as well, because its channel list never empties.

Remove the conn->binding gate so a connection always removes its
channels from every session on teardown.