PCI/MSI: Provide missing stub for pci_msix_can_alloc_dyn()

pci_msix_can_alloc_dyn() is not declared when CONFIG_PCI_MSI is disabled.

There is no existing user of pci_msix_can_alloc_dyn() but work is in
progress to change this. This work encounters the following error when
CONFIG_PCI_MSI is disabled:

  drivers/vfio/pci/vfio_pci_intrs.c:427:21: error: implicit declaration of function 'pci_msix_can_alloc_dyn' [-Werror=implicit-function-declaration]

Provide definition for pci_msix_can_alloc_dyn() in preparation for users
that need to compile when CONFIG_PCI_MSI is disabled.

[bhelgaas: Also reported by Arnd Bergmann <[email protected]> in
drivers/net/ethernet/mellanox/mlx5/core/pci_irq.c; added his Fixes: line]

Fixes: fb0a6a2 ("net/mlx5: Provide external API for allocating vectors")
Fixes: 3402636 ("PCI/MSI: Provide post-enable dynamic allocation interfaces for MSI-X")
Link: https://lore.kernel.org/oe-kbuild-all/[email protected]/
Link: https://lore.kernel.org/r/310ecc4815dae4174031062f525245f0755c70e2.1680119924.git.reinette.chatre@intel.com
Reported-by: kernel test robot <[email protected]>
Signed-off-by: Reinette Chatre <[email protected]>
Signed-off-by: Bjorn Helgaas <[email protected]>
Reviewed-by: Kuppuswamy Sathyanarayanan <[email protected]>
Cc: [email protected]	# v6.2+

Author: rchatre

include/linux/pci.h

@@ -1623,6 +1623,8 @@ pci_alloc_irq_vectors(struct pci_dev *dev, unsigned int min_vecs,
 					      flags, NULL);
 }
 
+static inline bool pci_msix_can_alloc_dyn(struct pci_dev *dev)
+{ return false; }
 static inline struct msi_map pci_msix_alloc_irq_at(struct pci_dev *dev, unsigned int index,
 						   const struct irq_affinity_desc *affdesc)
 {