ibmvnic: Enable scatter-gather support

This patch enables scatter gather support. Since there is no
HW/FW scatter-gather support at this time, the driver needs to
loop through each fragment and copy it to a contiguous, pre-mapped
buffer entry.

Signed-off-by: Thomas Falcon <tlfalcon@linux.vnet.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>