sctp: Tighten up locking around sctp_aloc_assoc()

All callers of sctp_aloc_assoc() mark the PCB as connected after a
successful call (for one-to-one-style sockets). In all cases this is
done without the PCB lock, so the PCB's flags can be corrupted. We also
do not atomically check whether a one-to-one-style socket is a listening
socket, which violates various assumptions in solisten_proto().

We need to hold the PCB lock across all of sctp_aloc_assoc() to fix
this. In order to do that without introducing lock order reversals, we
have to hold the global info lock as well.

So:

• Convert sctp_aloc_assoc() so that the inp and info locks are consistently held. It returns with the association lock held, as before.
• Fix an apparent bug where we failed to remove an association from a global hash if sctp_add_remote_addr() fails.
• sctp_select_a_tag() is called when initializing an association, and it acquires the global info lock. To avoid lock recursion, push locking into its callers.
• Introduce sctp_aloc_assoc_connected(), which atomically checks for a listening socket and sets SCTP_PCB_FLAGS_CONNECTED.

There is still one edge case in sctp_process_cookie_new() where we do
not update PCB/socket state correctly.

Reviewed by: tuexen
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D31908