io_uring: optimise io_import_iovec fixed path

Delay loading req->rw.{addr,len} in io_import_iovec until it's really
needed, so removing extra loads for the fixed path, which doesn't use
them.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/3cc48dd0c4f1a37c4ce9aab5784281a2d83ad8be.1634314022.git.asml.silence@gmail.com


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