fix: handle overwriting tags in syslinux ADV

This is (still) being used in Talos to handle upgrade rollbacks.

There were multiple problems with this code, and one of them leads to
panic if the tag is written multiple times without deletion:

```
github.com/siderolabs/talos/internal/app/machined/pkg/runtime/v1alpha1/bootloader/adv/syslinux.ADV.SetTagBytes({0xc00175bc00?, 0x1f11dbe?, 0xed4f4d?}, 0x0?, {0xc000afb7f0?, 0x400?, 0x0?})
/src/internal/app/machined/pkg/runtime/v1alpha1/bootloader/adv/syslinux/syslinux.go:125 +0x270
github.com/siderolabs/talos/internal/app/machined/pkg/runtime/v1alpha1/bootloader/adv/syslinux.ADV.SetTag(...)
/src/internal/app/machined/pkg/runtime/v1alpha1/bootloader/adv/syslinux/syslinux.go:95
github.com/siderolabs/talos/cmd/installer/pkg/install.(*Installer).Install(0xc0004374a0, 0x5)
/src/cmd/installer/pkg/install/install.go
```

The `uint8()` conversion was causing overflow and wrong index when ADV
real length is over 255.

Fix multiple writes of the same tag by deleting previous value first.

Signed-off-by: Andrey Smirnov <andrey.smirnov@talos-systems.com>

internal/app/machined/pkg/runtime/v1alpha1/bootloader/adv/syslinux/syslinux.go

@@ -101,6 +101,9 @@ func (a ADV) SetTagBytes(t uint8, val []byte) (ok bool) {
 		return false
 	}
 
+	// delete the tag if it exists
+	a.DeleteTag(t)
+
 	// Header is in first 8 bytes.
 	i := 8
 
@@ -116,13 +119,17 @@ func (a ADV) SetTagBytes(t uint8, val []byte) (ok bool) {
 			continue
 		}
 
+		// overflow check
+		if i+2+len(val) > AdvSize-4-2 {
+			return false
+		}
+
 		length := uint8(len(val))
 
 		a[i] = t
-
 		a[i+1] = length
 
-		copy(a[i+2:uint8(i+2)+length], val)
+		copy(a[i+2:i+2+int(length)], val)
 
 		ok = true
 

internal/app/machined/pkg/runtime/v1alpha1/bootloader/adv/syslinux/syslinux_test.go

@@ -324,3 +324,31 @@ func TestADV_tail(t *testing.T) {
 		})
 	}
 }
+
+func TestADV_overwrite(t *testing.T) {
+	buf := make([]byte, 2*AdvSize)
+
+	a, err := NewADV(bytes.NewReader(buf))
+	if err != nil {
+		t.Errorf("NewADV() failed: %s", err)
+	}
+
+	for i := 0; i < 1024; i++ {
+		if !a.SetTag(adv.Bootonce, "yes") {
+			t.Errorf("SetTag() failed")
+		}
+	}
+}
+
+func TestADV_many_tags(t *testing.T) {
+	buf := make([]byte, 2*AdvSize)
+
+	a, err := NewADV(bytes.NewReader(buf))
+	if err != nil {
+		t.Errorf("NewADV() failed: %s", err)
+	}
+
+	for i := uint8(1); i < 255; i++ {
+		a.SetTag(i, "xa")
+	}
+}