Since commit 636b21b5 ("PCI: Revoke mappings like devmem"), mmappable
sysfs entries have started to receive their f_mapping from the iomem
pseudo filesystem, so that CONFIG_IO_STRICT_DEVMEM is honored in sysfs
(and procfs) as well as in /dev/[k]mem.

This resulted in a userspace-visible regression:

1. Open a sysfs PCI resource file (eg. /sys/bus/pci/devices/*/resource0)
2. Use lseek(fd, 0, SEEK_END) to determine its size

Expected result: a PCI region size is returned.
Actual result: 0 is returned.

The reason is that PCI resource files residing in sysfs use
generic_file_llseek(), which relies on f_mapping->host inode to get the
file size. As f_mapping is now redefined, f_mapping->host points to an
anonymous zero-sized iomem_inode which has nothing to do with sysfs file
in question.

Implement a custom llseek method for sysfs PCI resources, which is
almost the same as proc_bus_pci_lseek() used for procfs entries.

This makes sysfs and procfs entries consistent with regards to seeking,
but also introduces userspace-visible changes to seeking PCI resources
in sysfs:

- SEEK_DATA and SEEK_HOLE are no longer supported;
- Seeking past the end of the file is prohibited while previously
  offsets up to MAX_NON_LFS were accepted (reading from these offsets
  was always invalid).
Signed-off-by: Valentine Sinitsyn <valesini@yandex-team.ru>
Acked-by: Bjorn Helgaas <bhelgaas@google.com>
Link: https://lore.kernel.org/r/20230925084013.309399-2-valesini@yandex-team.ruSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>