io_uring: keep table of pointers to ubufs

Instead of keeping a table of ubufs convert them into pointers to ubuf,
so we can atomically read one pointer and be sure that the content of
ubuf won't change.

Because it was already dynamically allocating imu->bvec, throw both
imu and bvec into a single structure so they can be allocated together.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/b96efa4c5febadeccf41d0e849ac099f4c83b0d3.1619356238.git.asml.silence@gmail.com


Signed-off-by: Jens Axboe <axboe@kernel.dk>