vfio: Drop vfio_file_iommu_group() stub to fudge around a KVM wart

Drop the vfio_file_iommu_group() stub and instead unconditionally declare
the function to fudge around a KVM wart where KVM tries to do symbol_get()
on vfio_file_iommu_group() (and other VFIO symbols) even if CONFIG_VFIO=n.

Ensuring the symbol is always declared fixes a PPC build error when
modules are also disabled, in which case symbol_get() simply points at the
address of the symbol (with some attributes shenanigans).  Because KVM
does symbol_get() instead of directly depending on VFIO, the lack of a
fully defined symbol is not problematic (ugly, but "fine").

   arch/powerpc/kvm/../../../virt/kvm/vfio.c:89:7:
   error: attribute declaration must precede definition [-Werror,-Wignored-attributes]
           fn = symbol_get(vfio_file_iommu_group);
                ^
   include/linux/module.h:805:60: note: expanded from macro 'symbol_get'
   #define symbol_get(x) ({ extern typeof(x) x __attribute__((weak,visibility("hidden"))); &(x); })
                                                              ^
   include/linux/vfio.h:294:35: note: previous definition is here
   static inline struct iommu_group *vfio_file_iommu_group(struct file *file)
                                     ^
   arch/powerpc/kvm/../../../virt/kvm/vfio.c:89:7:
   error: attribute declaration must precede definition [-Werror,-Wignored-attributes]
           fn = symbol_get(vfio_file_iommu_group);
                ^
   include/linux/module.h:805:65: note: expanded from macro 'symbol_get'
   #define symbol_get(x) ({ extern typeof(x) x __attribute__((weak,visibility("hidden"))); &(x); })
                                                                   ^
   include/linux/vfio.h:294:35: note: previous definition is here
   static inline struct iommu_group *vfio_file_iommu_group(struct file *file)
                                     ^
   2 errors generated.

Although KVM is firmly in the wrong (there is zero reason for KVM to build
virt/kvm/vfio.c when VFIO is disabled), fudge around the error in VFIO as
the stub is unnecessary and doesn't serve its intended purpose (KVM is the
only external user of vfio_file_iommu_group()), and there is an in-flight
series to clean up the entire KVM<->VFIO interaction, i.e. fixing this in
KVM would result in more churn in the long run, and the stub needs to go
away regardless.

Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202308251949.5IiaV0sz-lkp@intel.com
Closes: https://lore.kernel.org/oe-kbuild-all/202309030741.82aLACDG-lkp@intel.com
Closes: https://lore.kernel.org/oe-kbuild-all/202309110914.QLH0LU6L-lkp@intel.com
Link: https://lore.kernel.org/all/0-v1-08396538817d+13c5-vfio_kvm_kconfig_jgg@nvidia.com
Link: https://lore.kernel.org/all/20230916003118.2540661-1-seanjc@google.com
Cc: Nick Desaulniers <ndesaulniers@google.com>
Cc: Jason Gunthorpe <jgg@nvidia.com>
Tested-by: Michael Ellerman <mpe@ellerman.id.au>
Fixes: c1cce6d0

 ("vfio: Compile vfio_group infrastructure optionally")
Signed-off-by: Sean Christopherson <seanjc@google.com>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Link: https://lore.kernel.org/r/20231130001000.543240-1-seanjc@google.com
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>