KVM: debugfs: Return retval of simple_attr_open() if it fails

Although simple_attr_open() fails only with -ENOMEM with current code
base, it would be nicer to return retval of simple_attr_open() directly
in kvm_debugfs_open().

No functional change intended.

Signed-off-by: Hou Wenlong <houwenlong.hwl@antgroup.com>
Message-Id: <69d64d93accd1f33691b8a383ae555baee80f943.1665975828.git.houwenlong.hwl@antgroup.com>
Cc: stable@vger.kernel.org
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>