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

This reverts commit 79996b45.

Revert the change that restricts a send provided buffer to be zero, so
it will always consume the whole buffer. This is strictly needed for
partial consumption, as the send may very well be a subset of the
current buffer. In fact, that's the intended use case.

For non-incremental provided buffer rings, an application should set
sqe->len carefully to avoid the potential issue described in the
reverted commit. It is recommended that '0' still be set for len for
that case, if the application is set on maintaining more than 1 send
inflight for the same socket. This is somewhat of a nonsensical thing
to do.

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