do a single memdup_user in sctp_setsockopt

Hi all,

based on the review of Davids patch to do something similar I dusted off
the series I had started a few days ago to move the memdup_user or
copy_from_user from the inidividual sockopts into sctp_setsockopt,
which is done with one patch per option, so it might suit Marcelo's
taste a bit better.  I did not start any work on getsockopt.