sctp: cleanup locking for notifications

All notifications are now queued via sctp_ulp_notify(). Do
the locking of the inp read lock there and validate this in all
functions being used.
This is one step in avoiding race conditions when closing the
read end of an SCTP socket.

(cherry picked from commit f9425b3a85e9e211b61e11ce8115bf73674bdf49)