amd-xgbe: Rework the Rx path SKB allocation

Rework the SKB allocation so that all of the buffers of the first
descriptor are handled in the SKB allocation routine. After copying the
data in the header buffer (which can be just the header if split header
processing succeeded for header plus data if split header processing did
not succeed) into the SKB, check for remaining data in the receive
buffer. If there is data remaining in the receive buffer, add that as a
frag to the SKB. Once an SKB has been allocated, all other descriptors
are added as frags to the SKB.

Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
Signed-off-by: David S. Miller <davem@davemloft.net>