sctp: handle invalid error codes without calling BUG()

The sctp_sf_eat_auth() function is supposed to return enum sctp_disposition
values but if the call to sctp_ulpevent_make_authkey() fails, it returns
-ENOMEM.

This results in calling BUG() inside the sctp_side_effects() function.
Calling BUG() is an over reaction and not helpful.  Call WARN_ON_ONCE()
instead.

This code predates git.

Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>