io_uring/kbuf: drop WARN_ON_ONCE() from incremental length check

Partially based on commit 98b6fa62 upstream.

This can be triggered by userspace, so just drop it. The condition
is appropriately handled.

Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>