powerpc/radix: synchronize temporary kernel mappings

[mchoo]

Summary

The radix kremove implementation cleared a kernel PTE without
invalidating its TLB entry. Reusing crashdumpmap could therefore keep
accessing an old physical page, causing minidumps to contain repeated
stale page contents.

Invalidate the removed kernel mapping and synchronize newly installed
kernel PTEs before they are accessed.

Fixes: 65bbba25d214 ("powerpc64: Implement Radix MMU for POWER9 CPUs")
MFC after: 2 weeks
MFC to: stable/14, stable/15
Sponsored by: FreeBSD Foundation

Test Plan

With radix mmu minidump implementation that will be posted later, this works correctly.
Without this, debuggers show corrupted memory content, thus LLDB cannot read allproc,
stoppcbs, etc correctly and thus displays zero threads.

[mchoo]

ping @jhb for mentor approval

[mchoo]

ping @olce for mentor approval