Linux 6.15: mkdir now returns struct dentry *

The intent is that the filesystem may have a reference to an "old"
version of the new directory, eg if it was keeping it alive because a
remote NFS client still had it open.

We don't need anything like that, so this really just changes things so
we return error codes encoded in pointers.

Sponsored-by: https://despairlabs.com/sponsor/
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Reviewed-by: Pavel Snajdr <snajpa@snajpa.net>
Signed-off-by: Rob Norris <robn@despairlabs.com>
Closes #17229