iov_iter: overlay struct iovec and ubuf/len

iov_iter: overlay struct iovec and ubuf/len

Add an internal struct iovec that we can return as a pointer, with the
fields of the iovec overlapping with the ITER_UBUF ubuf and length
fields.

This allows doing:

struct iovec *vec = &iter->__ubuf_iovec;

interchangably with iter->iov for the first segment, enabling writing
code that deals with both without needing to check if we're dealing with
ITER_IOVEC or ITER_UBUF.

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