vfio/iommu_type1: initialize pgsize_bitmap in ->open

Ensure pgsize_bitmap is always valid by initializing it to PAGE_MASK
in vfio_iommu_type1_open and remove the now pointless update for
the external domain case in vfio_iommu_type1_attach_group, which was
just setting pgsize_bitmap to PAGE_MASK when only external domains
were attached.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20210924155705.4258-14-hch@lst.de


[aw: s/ULONG_MAX/PAGE_MASK/ per discussion in link]
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>