powerpc: fix unbalanced node refcount in check_kvm_guest()

When check_kvm_guest() succeeds in looking up a /hypervisor OF node, it
returns without performing a matching put for the lookup, leaving the
node's reference count elevated.

Add the necessary call to of_node_put(), rearranging the code slightly to
avoid repetition or goto.

Fixes: 107c55005f ("powerpc/pseries: Add KVM guest doorbell restrictions")
Signed-off-by: Nathan Lynch <nathanl@linux.ibm.com>
Reviewed-by: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
Reviewed-by: Tyrel Datwyler <tyreld@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210928124550.132020-1-nathanl@linux.ibm.com

arch/powerpc/kernel/firmware.c

@@ -31,11 +31,10 @@ int __init check_kvm_guest(void)
 	if (!hyper_node)
 		return 0;
 
-	if (!of_device_is_compatible(hyper_node, "linux,kvm"))
-		return 0;
-
-	static_branch_enable(&kvm_guest);
+	if (of_device_is_compatible(hyper_node, "linux,kvm"))
+		static_branch_enable(&kvm_guest);
 
+	of_node_put(hyper_node);
 	return 0;
 }
 core_initcall(check_kvm_guest); // before kvm_guest_init()