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)").