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)