Re: do a single memdup_user in sctp_setsockopt v2

From: Christoph Hellwig <hch@lst.de>
Date: Sun, 19 Jul 2020 09:21:37 +0200

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

This is all very mechanical and contained to the sockopt code of SCTP,
so I reviewed this a few times and applied it to net-next.

Thanks Christoph!