io_uring: Require zeroed sqe->len on provided-buffers send

When sending from a provided buffer, we set sr->len to be the smallest
between the actual buffer size and sqe->len.  But, now that we
disconnect the buffer from the submission request, we can get in a
situation where the buffers and requests mismatch, and only part of a
buffer gets sent.  Assume:

* buf[1]->len = 128; buf[2]->len = 256
* sqe[1]->len = 128; sqe[2]->len = 256

If sqe1 runs first, it picks buff[1] and it's all good. But, if sqe[2]
runs first, sqe[1] picks buff[2], and the last half of buff[2] is
never sent.

While arguably the use-case of different-length sends is questionable,
it has already raised confusion with potential users of this
feature. Let's make the interface less tricky by forcing the length to
only come from the buffer ring entry itself.

Fixes: ac5f71a3 ("io_uring/net: add provided buffer support for IORING_OP_SEND")
Signed-off-by: Gabriel Krisman Bertazi <krisman@suse.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>