io_uring/kbuf: reject zero sized provided buffers

This isn't fixing a real issue, but there's also zero point in going
through group and buffer setup, when the buffers are going to be
rejected once attempted to get used.

Cc: stable@vger.kernel.org
Reported-by:  <syzbot+58928048fd1416f1457c@syzkaller.appspotmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>