ixgb and e1000: Use new function for copybreak tests

There appears to be an off-by-1 defect in the maximum packet size
copied when copybreak is speified in these modules.

The copybreak module params are specified as:
"Maximum size of packet that is copied to a new buffer on receive"

The tests are changed from "< copybreak" to "<= copybreak"
and moved into new static functions for readability.

Signed-off-by: Joe Perches <joe@perches.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>