Linux 6.11: add compat macro for page_mapping()

Since the change to folios it has just been a wrapper anyway. Linux has
removed their wrapper, so we add one.

Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Rob Norris <robn@despairlabs.com>
Sponsored-by: https://despairlabs.com/sponsor/
Closes #16400