drm/amdkfd: Fix kfd process ref leaking when userptr unmapping

In the Linux kernel, the following vulnerability has been resolved: drm/amdkfd: Fix kfd process ref leaking when userptr unmapping kfd_lookup_process_by_pid hold the kfd process reference to ensure it doesn't get destroyed while sending the segfault event to user space. Calling kfd_lookup_process_by_pid as function parameter leaks the kfd process refcount and miss the NULL pointer check if app process is already destroyed.