net: thunderx: Use skb_add_rx_frag() for split buffer Rx pkts

Instead of a round about way of converting buffers to SKBs and
combining them into a frag list, use standard skb_add_rx_frag()
API to merge page fragments. This code is useful when incoming
packets are of size more than RCV_FRAG_LEN which is currently
set to 2048bytes.

Signed-off-by: Sunil Goutham <sgoutham@cavium.com>
Signed-off-by: David S. Miller <davem@davemloft.net>