crypto/ccp: Implement SNP x86 shutdown

crypto/ccp: Implement SNP x86 shutdown

The SEV firmware has support to disable SNP during an SNP_SHUTDOWN_EX command.
Verify that this support is available and set the flag so that SNP is disabled
when it is not being used.

In cases where SNP is disabled, skip the call to amd_iommu_snp_disable(), as
all of the IOMMU pages have already been made shared. Also skip the panic
case, since snp_shutdown() does IPIs.

Signed-off-by: Tycho Andersen (AMD) <tycho@kernel.org>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Reviewed-by: Tom Lendacky <thomas.lendacky@amd.com>
Acked-by: Herbert Xu <herbert@gondor.apana.org.au>
Link: https://patch.msgid.link/20260324161301.1353976-7-tycho@kernel.org