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

[olce]

Summary

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.

[imp]

This is good, though I'm thinking the XXX comment should maybe be expanded a word or five and the XXX removed. Up to you though.