xattr: switch to CLASS(fd)

In the Linux kernel, the following vulnerability has been resolved: fs/xattr: missing fdput() in fremovexattr error path In the Linux kernel, the fremovexattr() syscall calls fdget() to acquire a file reference but returns early without calling fdput() when strncpy_from_user() fails on the name argument. In multi-threaded processes where fdget() takes the slow path, this permanently leaks one file reference per call, pinning the struct file and associated kernel objects in memory. An unprivileged local user can exploit this to cause kernel memory exhaustion. The issue was inadvertently fixed by commit a71874379ec8 ("xattr: switch to CLASS(fd)").