misc: nsm: pin the module while the device is open

In the Linux kernel, the following vulnerability has been resolved: misc: nsm: pin the module while the device is open misc_open() installs a misc driver's file operations with fops_get(), which pins file_operations::owner before replacing the file's f_op. The NSM misc device leaves nsm_dev_fops.owner unset, so opening /dev/nsm does not take a module reference on the nsm driver. If the driver is built as a module, an open file descriptor can therefore survive rmmod of the module that provides its ioctl callbacks. A later ioctl through that descriptor can call into unloaded module text. Set nsm_dev_fops.owner to THIS_MODULE so the misc core holds the module while any /dev/nsm file descriptor is open, matching the lifetime expectation for the installed file operations.