sockets: repair sctp_peeloff(2)

The shim function soattach() may be passed a non-listening socket by SCTP.

NB: the change makes soattach() more hairy, but long term plan is that
this function goes away.

PR: 293010
Fixes: 64f7e3c9c178ab35cb1f8fdf791aec74ede6f6b2