kern/53875: ktrace.out starts with lots of zeroes

[uwe%NetBSD.org@localhost]

>Number:         53875
>Category:       kern
>Synopsis:       ktrace.out starts with lots of zeroes
>Confidential:   no
>Severity:       non-critical
>Priority:       medium
>Responsible:    kern-bug-people
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Tue Jan 15 15:55:00 +0000 2019
>Originator:     Valery Ushakov
>Release:        NetBSD-current
>Organization:
>Environment:
NetBSD nbvio 8.99.30 NetBSD 8.99.30 (GENERIC) #0: Mon Jan  7 03:02:09 MSK 2019  uwe@majava:/home/uwe/work/netbsd/cvs/src/sys/arch/amd64/compile/GENERIC amd64

>Description:
If you try to trace a process that is already running you get a
ktrace.out file with lots of leading zeroes.

>How-To-Repeat:
In one terminal start some long-running program, e.g.

$ rain -d 200

In another terminal find the pid of that test program and try to
trace it:

$ ktrace -p $pid; sleep 1; ktrace -p $pid
$ hexdump -C ktrace.out | sed 4q
00000000  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
*
00187590  00 20 00 00 00 01 00 02  00 2a 12 00 00 72 61 69  |. .......*...rai|
001875a0  6e 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |n...............|

>Fix: