Adjust libmemstat after r338899

[mjg]

Summary

libmemstat uses kvm to dig into kernel memory to get malloc stats. However, the way stats are kept is not versioned in any way and the recent change results in garbage results or crashes
e.g. using vmstat -m -M /dev/mem

The change simply updates the lib to match the kernel.

the issue was reported by yuripv

Test Plan

compared vmstat -m -M /dev/mem to vmstat -m

[kib]

I do not remember that we ever supported non-current kernel in the core/live kernel tools. In other words, I do not think that the versioning to support future compatibility is needed. If you want some assurance, use __FreeBSD_version, this is a rare case where the value is actually useful and can be applied on purpose.

[markj]

In D17490#373345, @kib wrote:

I do not remember that we ever supported non-current kernel in the core/live kernel tools.

I agree, in general we simply don't provide backwards compatibility for -M. Support for this option typically requires embedding kernel struct layout into userspace, and we have no versioning mechanism for that.

[mjg]

Just implement support for the updated layout.

[mjg]

The intent was not really to be compatible, but to provide the library a fighting chance to fail gracefully instead of crashing in case of potential future changes.

[markj]

lib/libmemstat/memstat_malloc.c

408 ↗

(On Diff #48989)

It looks like you should not be re-initializing j here.

[markj]

Looks ok to me otherwise.