sctp: Tighten up locking around sctp_aloc_assoc()

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