net: Take common prefetch code structure into a function

Many device drivers use the same prefetch code structure to
deal with small L1 cacheline size.
Take this code into a function and call it from the drivers.

Suggested-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: Tariq Toukan <tariqt@mellanox.com>
Reviewed-by: Saeed Mahameed <saeedm@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>