liquidio: Adjust a NULL pointer handling path in lio_vf_rep_copy_packet

In lio_vf_rep_copy_packet() pg_info->page is compared to a NULL value,
but then it is unconditionally passed to skb_add_rx_frag() which looks
strange and could lead to null pointer dereference.

lio_vf_rep_copy_packet() call trace looks like:
	octeon_droq_process_packets
	 octeon_droq_fast_process_packets
	  octeon_droq_dispatch_pkt
	   octeon_create_recv_info
	    ...search in the dispatch_list...
	     ->disp_fn(rdisp->rinfo, ...)
	      lio_vf_rep_pkt_recv(struct octeon_recv_info *recv_info, ...)
In this path there is no code which sets pg_info->page to NULL.
So this check looks unneeded and doesn't solve potential problem.
But I guess the author had reason to add a check and I have no such card
and can't do real test.
In addition, the code in the function liquidio_push_packet() in
liquidio/lio_core.c does exactly the same.

Based on this, I consider the most acceptable compromise solution to
adjust this issue by moving skb_add_rx_frag() into conditional scope.

Found by Linux Verification Center (linuxtesting.org) with SVACE.

Fixes: 1f233f32 ("liquidio: switchdev support for LiquidIO NIC")
Signed-off-by: Aleksandr Mishin <amishin@t-argos.ru>
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>