sctp: Fix lock recursion in sctp_swap_inpcb_for_listen()

[markj]

Summary

After commit bd4a39cc93d9 we now hold the global inp info lock across
the call to sctp_swap_inpcb_for_listen(), which attempts to acquire it
again. Since sctp_swap_inpcb_for_listen()'s sole caller is
sctp_listen(), we can simply change it to not try to acquire the lock.

Fixes: bd4a39cc93d9 ("socket: Properly interlock when transitioning to a listening socket")
Reported by: syzkaller