KVM: SVM: Open code handling of unexpected exits in svm_invoke_exit_handler()

KVM: SVM: Open code handling of unexpected exits in svm_invoke_exit_handler()

Fold svm_check_exit_valid() and svm_handle_invalid_exit() into their sole
caller, svm_invoke_exit_handler(), as having tiny single-use helpers makes
the code unncessarily difficult to follow.  This will also allow for
additional cleanups in svm_invoke_exit_handler().

No functional change intended.

Suggested-by: Yosry Ahmed <yosry.ahmed@linux.dev>
Reviewed-by: Yosry Ahmed <yosry.ahmed@linux.dev>
Reviewed-by: Pankaj Gupta <pankaj.gupta@amd.com>
Link: https://patch.msgid.link/20251230211347.4099600-3-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>