net/enic: support scattered Rx

net/enic: support scattered Rx

For performance reasons, this patch uses 2 VIC RQs per RQ presented to
DPDK.

The VIC requires that each descriptor be marked as either a start of
packet (SOP) descriptor or a non-SOP descriptor.  A one RQ solution
requires skipping descriptors when receiving small packets and results
in bad performance when receiving many small packets.

The 2 RQ solution makes use of the VIC feature that allows a receive
on primary queue to 'spill over' into another queue if the receive is
too large to fit in the buffer assigned to the descriptor on the
primary queue.  This means that there is no skipping of descriptors
when receiving small packets and results in much better performance.

Signed-off-by: Nelson Escobar <neescoba@cisco.com>
Reviewed-by: John Daley <johndale@cisco.com>