ionic: use fewer inits on the buf_info struct

Based on Alex's review notes on [1], we don't need to write
to the buf_info elements as often, and can tighten up how they
are used.  Also, use prefetchw() to warm up the page struct
for a later get_page().

[1] https://lore.kernel.org/netdev/CAKgT0UfyjoAN7LTnq0NMZfXRv4v7iTCPyAb9pVr3qWMhop_BVw@mail.gmail.com/



Signed-off-by: Shannon Nelson <snelson@pensando.io>
Signed-off-by: David S. Miller <davem@davemloft.net>