vfio/pci: Use unmap_mapping_range()

With the vfio device fd tied to the address space of the pseudo fs
inode, we can use the mm to track all vmas that might be mmap'ing
device BARs, which removes our vma_list and all the complicated lock
ordering necessary to manually zap each related vma.

Note that we can no longer store the pfn in vm_pgoff if we want to use
unmap_mapping_range() to zap a selective portion of the device fd
corresponding to BAR mappings.

This also converts our mmap fault handler to use vmf_insert_pfn()
because we no longer have a vma_list to avoid the concurrency problem
with io_remap_pfn_range().  The goal is to eventually use the vm_ops
huge_fault handler to avoid the additional faulting overhead, but
vmf_insert_pfn_{pmd,pud}() need to learn about pfnmaps first.

Also, Jason notes that a race exists between unmap_mapping_range() and
the fops mmap callback if we were to call io_remap_pfn_range() to
populate the vma on mmap.  Specifically, mmap_region() does call_mmap()
before it does vma_link_file() which gives a window where the vma is
populated but invisible to unmap_mapping_range().

Suggested-by: Jason Gunthorpe <jgg@nvidia.com>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Link: https://lore.kernel.org/r/20240530045236.1005864-3-alex.williamson@redhat.com
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>