[should've been added in 2016] stale comment in finish_open() nonwithstanding,
failure exits in ->atomic_open() instances should *NOT* fput() the file,
no matter what. Everything is handled by the caller.
+
+---
+
+**mandatory**
+
+clone_private_mount() returns a longterm mount now, so the proper destructor of
+its result is kern_unmount() or kern_unmount_array().