Linux 6.17: d_set_d_op() is no longer available

We only have extremely narrow uses, so move it all into a single
function that does only what we need, with and without d_set_d_op().

Sponsored-by: https://despairlabs.com/sponsor/
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Signed-off-by: Rob Norris <robn@despairlabs.com>
Closes #17621