Report offset relative to the backing object for kinfo_vmentry structures.

For the pathname reported in kinfo_vmentry structures (kve_path), the
sysctl handlers walk the object chain to find the bottom-most VM object.
This permits a COW mapping of a file with dirty pages to report the
pathname of the originally mapped file. Do the same for the object
offset (kve_offset) computing a cumulative offset during the same object
walk so that the reported offset is relative to the reported pathname.

Note that ptrace(PT_VM_ENTRY) already returns a cumulative offset
rather than the raw offset of the VM map entry.

Note also that this does not affect procstat -v output (even structured
output) since that output does not include the kve_offset field.

Reviewed by: kib
MFC after: 2 weeks
Sponsored by: DARPA / AFRL
Differential Revision: https://reviews.freebsd.org/D13767