mailbox: imx: Add a channel shutdown field

In the Linux kernel, the following vulnerability has been resolved: mailbox: imx: Add a channel shutdown field sashiko complained about possible teardown problem. The scenario CPU 0 CPU 1 imx_mu_isr() imx_mu_shutdown() imx_mu_xcr_rmw(priv, IMX_MU_RCR, 0, IMX_MU_xCR_RIEn(priv->dcfg->type, cp->idx)); imx_mu_specific_rx() imx_mu_xcr_rmw(priv, IMX_MU_RCR, IMX_MU_xCR_RIEn(priv->dcfg->type, 0), 0); free_irq() The RX event remains enabled because in this short window the RX event was disabled in ->shutdown() while the interrupt was active and then enabled again by the ISR while ->shutdown waited in free_irq(). This race requires timing and if happens can be problematic on shared handlers if the "removed" channel triggers an interrupt. In this case the irq-core will shutdown the interrupt with the "nobody cared" message. Introduce imx_mu_con_priv::shutdown to signal that the channel is shutting down. This flag is set with the lock held (by imx_mu_xcr_clr_shut()). The unmask side uses imx_mu_xcr_set_act() which only enables the event if the channel has not been shutdown and serialises on the same lock.