smbios: buffer overflow when zeroing entry point

A SMBIOS 3 entry point has a different length than an SMBIOS 2.1 entry
point.

Fixes: 70924294 ("smbios: Use SMBIOS 3.0 to support an address above 4GB")
Fixes: 1c5f6fa3 ("smbios: Drop support for SMBIOS2 tables")
Addresses-Coverity-ID: 477212 ("Wrong sizeof argument")
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>