persistent modify PWM, report IOCTL error, and hung

[fly-studio]

Describe the bug

run code, and after some indeterminate time, exit and report:

bcm283x-gpio (GPIO21): videocore: failed request to allocate memory: failed to send IOCTL: connection timed out
exit status 1

restart and hung when call PWM(duty > 0) , Ctrl+C will not works. AND raspberry pi cannot restart normally via init 6, MUST shutdown the power

BUT RPi.GPIO(python) or wiringpi(C) can works normally with the same GPIO after hanged

To Reproduce Steps to reproduce the behavior:

1. Run program Pulse light

   
   package main

import ( "fmt" "log" "periph.io/x/conn/v3/gpio" "periph.io/x/conn/v3/physic" "periph.io/x/host/v3" "periph.io/x/host/v3/rpi" "time" )

func main() {

if _, err := host.Init(); err != nil {
    panic(err.Error())
}

p := rpi.P1_40

i := 0
plus := true
for  {
    fmt.Printf("%d\n", i)
    if err := p.PWM(gpio.DutyMax * gpio.Duty(i) / 100, 1000 * physic.Hertz); err != nil {
        log.Fatal(err)
    }
    time.Sleep(10 * time.Millisecond)
    if plus {
        i++
    } else {
        i--
    }
    if i > 100 {
        plus = !plus
        i = 100
    }
    if i < 0 {
        plus = !plus
        i = 0
    }
            if err := p.Halt(); err != nil { // release gpu memory
        log.Fatal(err)
    }
}

}

2. Run it.
3. See error

**Expected behavior**

An pulse light effect

**Platform (please complete the following information):**
 - OS: OpenFans aarch64 2.0 (https://github.com/openfans-community-offical/Debian-Pi-Aarch64)
 - Board:  Raspberry Pi 4B 4G

**Additional context**

[fly-studio]

I read the implement of startPWMbyDMA, I think a memory leak was caused by allocateCB to /dev/vcio mailboxTx32(mbReleaseMemory, m.handle) maybe not work correctly

It'll not hung when useDMA=false,

so GPIO_13 GPIO_19 can works without hung

useDMA := false
    switch p.number {
    case 12, 40: // PWM0 alt0: disabled
        useDMA = true
    case 13, 41, 45: // PWM1
        f = alt0
    case 18: // PWM0 alt5: disabled
        useDMA = true
    case 19: // PWM1
        f = alt5
    default:
        useDMA = true
    }