Use a plain array for any group ID that's less than 64, and punt
anything beyond that to an xarray. 64 fits in a page even for 4KB
page sizes and with the planned additions.

This makes the expected group usage faster by avoiding a hash and lookup
to find our list, and it uses less memory upfront by not allocating any
memory for provided buffers unless it's actually being used.
Suggested-by: Pavel Begunkov <asml.silence@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>