x86/msi: fix locking for SR-IOV devices

x86/msi: fix locking for SR-IOV devices

In commit 4f78438b45e2 ("vpci: use per-domain PCI lock to protect vpci
structure") a lock was moved from allocate_and_map_msi_pirq() to the
caller and changed from pcidevs_lock() to read_lock(&d->pci_lock).
However, one call path wasn't updated to reflect the change, leading to
a failed assertion observed under the following conditions:

* PV dom0
* Debug build (CONFIG_DEBUG=y) of Xen
* There is an SR-IOV device in the system with one or more VFs enabled
* Dom0 has loaded the driver for the VF and enabled MSI-X

(XEN) Assertion 'd || pcidevs_locked()' failed at drivers/passthrough/pci.c:535
(XEN) ----[ Xen-4.20-unstable  x86_64  debug=y  Not tainted ]----
...
(XEN) Xen call trace:
(XEN)    [<ffff82d040284da8>] R pci_get_pdev+0x4c/0xab
(XEN)    [<ffff82d040344f5c>] F arch/x86/msi.c#read_pci_mem_bar+0x58/0x272
(XEN)    [<ffff82d04034530e>] F arch/x86/msi.c#msix_capability_init+0x198/0x755
(XEN)    [<ffff82d040345dad>] F arch/x86/msi.c#__pci_enable_msix+0x82/0xe8
(XEN)    [<ffff82d0403463e5>] F pci_enable_msi+0x3f/0x78
(XEN)    [<ffff82d04034be2b>] F map_domain_pirq+0x2a4/0x6dc
(XEN)    [<ffff82d04034d4d5>] F allocate_and_map_msi_pirq+0x103/0x262
(XEN)    [<ffff82d04035da5d>] F physdev_map_pirq+0x210/0x259
(XEN)    [<ffff82d04035e798>] F do_physdev_op+0x9c3/0x1454
(XEN)    [<ffff82d040329475>] F pv_hypercall+0x5ac/0x6af
(XEN)    [<ffff82d0402012d3>] F lstar_enter+0x143/0x150

In read_pci_mem_bar(), the VF obtains the struct pci_dev pointer for its
associated PF to access the vf_rlen array. This array is initialized in
pci_add_device() and is only populated in the associated PF's struct
pci_dev.

Access the vf_rlen array via the link to the PF, and remove the
troublesome call to pci_get_pdev().

Fixes: 4f78438b45e2 ("vpci: use per-domain PCI lock to protect vpci structure")
Reported-by: Teddy Astie <teddy.astie@vates.tech>
Signed-off-by: Stewart Hildebrand <stewart.hildebrand@amd.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>