do a single memdup_user in sctp_setsockopt v2

Hi all,

here is a resend of my series to lift the copy_from_user out of the
individual sctp sockopt handlers into the main sctp_setsockopt
routine.


Changes since v1:
 - fixes a few sizeof calls.
 - use memzero_explicit in sctp_setsockopt_auth_key instead of special
   casing it for a kzfree in the caller
 - remove some minor cleanups from sctp_setsockopt_autoclose to keep
   it closer to the existing version
 - add another little only vaguely related cleanup patch