sctp: fix socket type created by sctp_peeloff()

When calling sctp_peeloff() on a SOCK_SEQPACKET socket, the created
and returned socket has the type SOCK_STREAM.
This is specified in section 9.2 of RFC 6458.

Reported by: Xin Long
Event: Wiesbaden Hackathon 2026

(cherry picked from commit d195b3783fa4de5c1a95f6d95eb9444abce6778b)