[PATCH] PCI: Fix build error when CONFIG_PCI_MSI disabled

[Reinette Chatre <reinette.chatre@intel.com>]

pci_msix_alloc_irq_at() and pci_msix_free_irq() are not
declared when CONFIG_PCI_MSI is disabled.

Users of these two calls do not yet exist but when users
do appear (shown below is an attempt to use the new API
in vfio-pci) the following errors will be encountered when
compiling with CONFIG_PCI_MSI disabled:
drivers/vfio/pci/vfio_pci_intrs.c:461:4: error: implicit declaration of\
        function 'pci_msix_free_irq' is invalid in C99\
        [-Werror,-Wimplicit-function-declaration]
                           pci_msix_free_irq(pdev, msix_map);
                           ^
   drivers/vfio/pci/vfio_pci_intrs.c:461:4: note: did you mean 'pci_ims_free_irq'?
   include/linux/pci.h:2516:6: note: 'pci_ims_free_irq' declared here
   void pci_ims_free_irq(struct pci_dev *pdev, struct msi_map map);
        ^
drivers/vfio/pci/vfio_pci_intrs.c:511:15: error: implicit declaration of\
        function 'pci_msix_alloc_irq_at' is invalid in C99\
        [-Werror,-Wimplicit-function-declaration]
                   msix_map = pci_msix_alloc_irq_at(pdev, vector, NULL);
                                      ^
   drivers/vfio/pci/vfio_pci_intrs.c:511:15: note: did you mean 'pci_ims_alloc_irq'?
   include/linux/pci.h:2514:16: note: 'pci_ims_alloc_irq' declared here
   struct msi_map pci_ims_alloc_irq(struct pci_dev *pdev,\
                                    union msi_instance_cookie *icookie,

Provide definitions for pci_msix_alloc_irq_at() and pci_msix_free_irq() in
preparation for users that need to compile when CONFIG_PCI_MSI is
disabled.

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
---

checkpatch.pl warns about the usage of -ENOSYS but it does appear
to be the custom.

 include/linux/pci.h | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/include/linux/pci.h b/include/linux/pci.h
index adffd65e84b4..448482d1c4fe 100644
--- a/include/linux/pci.h
+++ b/include/linux/pci.h
@@ -1621,6 +1621,19 @@ pci_alloc_irq_vectors(struct pci_dev *dev, unsigned int min_vecs,
 					      flags, NULL);
 }
 
+static inline struct msi_map
+pci_msix_alloc_irq_at(struct pci_dev *dev, unsigned int index,
+		      const struct irq_affinity_desc *affdesc)
+{
+	struct msi_map map = { .index = -ENOSYS };
+
+	return map;
+}
+
+static inline void pci_msix_free_irq(struct pci_dev *pdev, struct msi_map map)
+{
+}
+
 static inline void pci_free_irq_vectors(struct pci_dev *dev)
 {
 }
-- 
2.34.1