io_uring: inline io_import_iovec()

Inline io_import_iovec() and leave only its former __io_import_iovec()
renamed to the original name. That makes it more obious what is reused in
io_read/write().

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>