KVM: SEV: Lock all vCPUs when synchronzing VMSAs for SNP launch finish

KVM: SEV: Lock all vCPUs when synchronzing VMSAs for SNP launch finish

Lock all vCPUs when synchronizing and encrypting VMSAs for SNP guests, as
allowing userspace to manipulate and/or run a vCPU while its state is being
synchronized would at best corrupt vCPU state, and at worst crash the host
kernel.

Opportunistically assert that vcpu->mutex is held when synchronizing its
VMSA (the SEV-ES path already locks vCPUs).

Fixes: ad27ce155566 ("KVM: SEV: Add KVM_SEV_SNP_LAUNCH_FINISH command")
Cc: stable@vger.kernel.org
Link: https://patch.msgid.link/20260310234829.2608037-6-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>