io_uring: don't store bgid in req->buf_index

io_uring: don't store bgid in req->buf_index

Pass buffer group id into the rest of helpers via struct buf_sel_arg
and remove all reassignments of req->buf_index back to bgid. Now, it
only stores buffer indexes, and the group is provided by callers.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/3ea9fa08113ecb4d9224b943e7806e80a324bdf9.1743437358.git.asml.silence@gmail.com
Link: https://lore.kernel.org/io-uring/0c01d76ff12986c2f48614db8610caff8f78c869.1743500909.git.asml.silence@gmail.com/
[axboe: fold in patch from second link]
Signed-off-by: Jens Axboe <axboe@kernel.dk>