io_uring: inline io_iter_do_read()

There are only two calls in source code of io_iter_do_read(), the
function is small and pretty hot though is failed to get inlined.
Makr it as inline.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/25a26dae7660da73fbc2244b361b397ef43d3caf.1623634182.git.asml.silence@gmail.com


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