debugfs: fix missing mutex_destroy() in short_fops case

we need that in ->real_fops == NULL, ->short_fops != NULL case

Fixes: 8dc6d81c "debugfs: add small file operations for most files"
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Link: https://lore.kernel.org/r/20241229081223.3193228-1-viro@zeniv.linux.org.uk


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>