smbios: Use smbios_next() at smbios_romfile_setup()

smbios: Use smbios_next() at smbios_romfile_setup()

Use smbios_next() instead of smbios_21_next(), to make the code
more generic and reusable for SMBIOS 3.0 support.

Note that `qtables_len` is initialized to `ftables->size` instead
of `ep.structure_table_length` now, but both fields are
guaranteed to have exactly the same value.

Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>