libsa: smbios: Allow to run smbios_probe() multiple times

This is in preparation for modifying the EFI loader to favor a v3 table
if present.

As an impact, caller smbios_match() has been changed so that it only
calls smbios_probe() with NULL (non-EFI discovery) once.

While here, expand the original XXXRP comment in smbios_match().

Reviewed by: imp, markj
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D49289

(cherry picked from commit c340797e08d8c20983ea57de88992f988fe7bb9b)