x86/sev: Disable KMSAN for memory encryption TUs

Instrumenting sev.c and mem_encrypt_identity.c with KMSAN will result in
a triple-faulting kernel. Some of the code is invoked too early during
boot, before KMSAN is ready.

Disable KMSAN instrumentation for the two translation units.

  [ bp: Massage commit message. ]

Signed-off-by: Changbin Du <changbin.du@huawei.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Link: https://lore.kernel.org/r/20240308044401.1120395-1-changbin.du@huawei.com