KVM: SVM: Use guard(mutex) to simplify SNP AP Creation error handling

KVM: SVM: Use guard(mutex) to simplify SNP AP Creation error handling

Use guard(mutex) in sev_snp_ap_creation() and modify the error paths to
return directly instead of jumping to a common exit point.

No functional change intended.

Reviewed-by: Tom Lendacky <thomas.lendacky@amd.com>
Reviewed-by: Pankaj Gupta <pankaj.gupta@amd.com>
Link: https://lore.kernel.org/r/20250227012541.3234589-8-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>