ktrace: Fix uninitialized memory disclosure

The sockaddr passed to ktrcapfail() may be smaller than
sizeof(struct sockaddr), and the trailing bytes in the sockaddr
structure will be uninitialized, whereupon they get copied out to
userspace.

PR: 283673
Reviewed by: jfree, emaste
Reported by: Yichen Chai <yichen.chai@gmail.com>
Reported by: Zhuo Ying Jiang Li <zyj20@cl.cam.ac.uk>
Fixes: 9bec84131215 ("ktrace: Record detailed ECAPMODE violations")
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D48499