efi/libstub: smbios: Use length member instead of record struct size

The type 1 SMBIOS record happens to always be the same size, but there
are other record types which have been augmented over time, and so we
should really use the length field in the header to decide where the
string table starts.

Fixes: 550b33cf ("arm64: efi: Force the use of ...")
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>