This is needed to prevent sk_user_data being overwritten.
The check is done under the callback lock. This should prevent
a socket from being attached twice to a KCM mux. It also prevents
a socket from being attached for other use cases of sk_user_data
as long as the other cases set sk_user_data under the lock.
Followup work is needed to unify all the use cases of sk_user_data
to use the same locking.

Reported-by: syzbot+114b15f2be420a8886c3@syzkaller.appspotmail.com
Fixes: ab7ac4eb ("kcm: Kernel Connection Multiplexor module")
Signed-off-by: NTom Herbert <tom@quantonium.net>
Reviewed-by: NEric Dumazet <edumazet@google.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>