net: limit scope of a skb_zerocopy_iter_stream var

skb_zerocopy_iter_stream() only uses @orig_uarg in the !link_skb path,
and we can move the local variable in the appropriate block.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Reviewed-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>