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

(cherry picked from commit 5b86888bae651e54ccc0adde0ed897ec1c1e0d45)