libgeom: Fix segfault in 32-on-64 case

[des]

Summary

We were using strtoul() to parse object identifiers, which are kernel
pointers. This works fine as long as the kernel and userland match,
but in a 32-bit libgeom on a 64-bit kernel this will return ULONG_MAX
for all objects, resulting in memory corruption when we later pick the
wrong object while resolving consumer-producer references.

MFC after: 1 week
PR: 292127

[markmi_dsl-only.net]

Looking at the history, a fix in this area was tried 13 years ago and reverted for breaking devstat use. 12 years ago the below type of fix was committed instead and then reverted, sighting lessons learned from the prior attempt.

devstat may need to be tested with this change in place.

[emaste]

We're still truncating to a 32-bit void * so the opportunity for a bugs remain (even if in practice it's essentially impossible). I think it's at least worth a comment in the src explaining why the truncation is acceptable.