firewire: nosy: ensure user_length is taken into account when fetching packet contents

In the Linux kernel, the following vulnerability has been resolved: firewire: nosy: ensure user_length is taken into account when fetching packet contents Ensure that packet_buffer_get respects the user_length provided. If the length of the head packet exceeds the user_length, packet_buffer_get will now return 0 to signify to the user that no data were read and a larger buffer size is required. Helps prevent user space overflows.