net: sctp: only warn in proc_sctp_do_alpha_beta if write

Only warn if the value is written to alpha or beta. We don't care
emitting a one-time warning when only reading it.

Reported-by: Jiri Pirko <jpirko@redhat.com>
Signed-off-by: Daniel Borkmann <dborkman@redhat.com>
Reviewed-by: Jiri Pirko <jiri@resnulli.us>
Signed-off-by: David S. Miller <davem@davemloft.net>