dlm: replace BUG_ON with a less severe handling

BUG_ON() is a severe action for this case, specially now that DLM with
SCTP will use 1 socket per association. Instead, we can just close the
socket on this error condition and return from the function.

Also move the check to an earlier stage as it won't change and thus we
can abort as soon as possible.

Although this issue was reported when still using SCTP with 1-to-many
API, this cleanup wouldn't be that simple back then because we couldn't
close the socket and making sure such event would cease would be hard.
And actually, previous code was closing the association, yet SCTP layer
is still raising the new data event. Probably a bug to be fixed in SCTP.

Reported-by:  <tan.hu@zte.com.cn>
Signed-off-by: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
Signed-off-by: David Teigland <teigland@redhat.com>