smbios: use struct_table_length to get SMBIOS 2.1 total table length

The current code convert the SMBIOS 2.1 entry point structure to
SMBIOS 3.0 entry point structure. The max_struct_size member in
SMBIOS 2.1 entry point structure indicates
"Size of the largest SMBIOS structure, in bytes".
We need to use struct_table_length instead.

Fixes: 1c5aab80 ("smbios: copy QEMU tables")
Signed-off-by: Masahisa Kojima <masahisa.kojima@linaro.org>
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>