[PATCH] PCI: designware: add a check of msi_desc in irqchip

[cao.zou]

----------------------------------------------------
Summary:  PCI: designware: add a check of msi_desc in irqchip
Reviewers: paul
----------------------------------------------------

Upstream Project Name: linux-kernel
Upstream Project URL: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/
Applies to: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/
Brief Description: Incorrect mask bits leave device stalled after flood ping.
Will Submit to: shan lai <jiangshanlai@xxxxxxxxx>, linux-kernel@xxxxxxxxxxxxxxx
Origin of patch: Discovered by me while working on common_pc
IP content: None, trivial obvious fix.

Comments:
---------

this issue is reproduced in imx7d-sdb when running the kdump with pluging Intel 1030 PCI,
the root caused is machine_kexec_mask_interrupts will force to mask every exist irq_desc
include msi irq.

background:
msi irq mask is based on msi_desc, it is bound only in msi irq setup by pci driver.
for others not request msi irqs by pci driver, its msi_desc is null, so crash in
machine_kexec_mask_interrupts.