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

[ Upstream commit 34343eb0 ]

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>
Signed-off-by: Sasha Levin <sashal@kernel.org>