io_uring: simplify selected buf handling

As selected buffers are now stored in a separate field in a request, get
rid of rw/recv specific helpers and simplify the code.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/bd4a866d8d91b044f748c40efff9e4eacd07536e.1638714983.git.asml.silence@gmail.com


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