The fallback for full dumps without PT_DUMP_DELTA resolves kernbase,
then reuses the same namelist entry to probe for kernphys. A failed
kernphys lookup clears n_value, so the later assignment loses kernbase
and the bootstrap read of KPML4phys uses the wrong address.

Save kernbase immediately after its successful lookup, before reusing
the namelist entry. Keep the existing rejection of relocatable kernels
whose physical placement cannot be recovered from a legacy dump.

Fixes: 308659acbff3 ("libkvm/kvm_amd64: account for relocatable kernels")
MFC after: 2 weeks
Sponsored by: FreeBSD Foundation