arm64 kvm: fix panic in kvm.dropPageTables

Related with issue #3019, #4056.
When running hello-world with gvisor-kvm, there is panic when exits:

"
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x3c0 pc=0x7c3f18]

goroutine 284 [running]:
... ...
gvisor.dev/gvisor/pkg/sentry/platform/kvm.(*machine).dropPageTables(0x4000166840, 0x400032a040)
    pkg/sentry/platform/kvm/machine_arm64.go:111 +0x88 fp=0x4000479e00 sp=0x4000479da0 pc=0x7c3f18
"

Also make dropPageTables() arch independent.

pkg/sentry/platform/kvm/machine.go

@@ -469,6 +469,19 @@ func (m *machine) newDirtySet() *dirtySet {
 	}
 }
 
+// dropPageTables drops cached page table entries.
+func (m *machine) dropPageTables(pt *pagetables.PageTables) {
+	m.mu.Lock()
+	defer m.mu.Unlock()
+
+	// Clear from all PCIDs.
+	for _, c := range m.vCPUsByID {
+		if c != nil && c.PCIDs != nil {
+			c.PCIDs.Drop(pt)
+		}
+	}
+}
+
 // lock marks the vCPU as in user mode.
 //
 // This should only be called directly when known to be safe, i.e. when

pkg/sentry/platform/kvm/machine_amd64.go

@@ -84,19 +84,6 @@ const (
 	poolPCIDs = 8
 )
 
-// dropPageTables drops cached page table entries.
-func (m *machine) dropPageTables(pt *pagetables.PageTables) {
-	m.mu.Lock()
-	defer m.mu.Unlock()
-
-	// Clear from all PCIDs.
-	for _, c := range m.vCPUsByID {
-		if c != nil && c.PCIDs != nil {
-			c.PCIDs.Drop(pt)
-		}
-	}
-}
-
 // initArchState initializes architecture-specific state.
 func (c *vCPU) initArchState() error {
 	var (

pkg/sentry/platform/kvm/machine_arm64.go

@@ -100,19 +100,6 @@ func availableRegionsForSetMem() (phyRegions []physicalRegion) {
 	return phyRegions
 }
 
-// dropPageTables drops cached page table entries.
-func (m *machine) dropPageTables(pt *pagetables.PageTables) {
-	m.mu.Lock()
-	defer m.mu.Unlock()
-
-	// Clear from all PCIDs.
-	for _, c := range m.vCPUsByID {
-		if c.PCIDs != nil {
-			c.PCIDs.Drop(pt)
-		}
-	}
-}
-
 // nonCanonical generates a canonical address return.
 //
 //go:nosplit