Allow uvideo_mmap_queue() to fail gracefully when the mmap queue is full;

Allow uvideo_mmap_queue() to fail gracefully when the mmap queue is full;
found the hard way by sebastia@ four years ago, and I'd been sitting on that
diff since.

The initial diff was more aggressive and would free the mmap queue upon error,
but jakemsr@ had objections against this behaviour; this diff only fails
gracefully instead of panic'ing.