PCI: Also set up legacy files only after sysfs init

We are already doing this for all the regular sysfs files on PCI
devices, but not yet on the legacy io files on the PCI buses. Thus far
no problem, but in the next patch I want to wire up iomem revoke
support. That needs the vfs up and running already to make sure that
iomem_get_mapping() works.

Wire it up exactly like the existing code in
pci_create_sysfs_dev_files(). Note that pci_remove_legacy_files()
doesn't need a check since the one for pci_bus->legacy_io is
sufficient.

An alternative solution would be to implement a callback in sysfs to
set up the address space from iomem_get_mapping() when userspace calls
mmap(). This also works, but Greg didn't really like that just to work
around an ordering issue when the kernel loads initially.

v2: Improve commit message (Bjorn)

Acked-by: Bjorn Helgaas <bhelgaas@google.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Cc: Stephen Rothwell <sfr@canb.auug.org.au>
Cc: Jason Gunthorpe <jgg@ziepe.ca>
Cc: Kees Cook <keescook@chromium.org>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: John Hubbard <jhubbard@nvidia.com>
Cc: Jérôme Glisse <jglisse@redhat.com>
Cc: Jan Kara <jack@suse.cz>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: linux-mm@kvack.org
Cc: linux-arm-kernel@lists.infradead.org
Cc: linux-samsung-soc@vger.kernel.org
Cc: linux-media@vger.kernel.org
Cc: Bjorn Helgaas <bhelgaas@google.com>
Cc: linux-pci@vger.kernel.org
Link: https://patchwork.freedesktop.org/patch/msgid/20210205133632.2827730-1-daniel.vetter@ffwll.ch