gianfar: Do not reuse pages from emergency reserve

A driver using dev_alloc_page() must not reuse a page that had to
use emergency memory reserve.

Otherwise all packets using this page will be immediately dropped,
unless for very specific sockets having SOCK_MEMALLOC bit set.

This issue might be hard to debug, because only a fraction of the RX
ring buffer would suffer from drops.

Fixes: 75354148 ("gianfar: Add paged allocation and Rx S/G")
Signed-off-by: NEric Dumazet <edumazet@google.com>
Cc: Claudiu Manoil <claudiu.manoil@freescale.com>
Acked-by: NClaudiu Manoil <claudiu.manoil@nxp.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/ethernet/freescale/gianfar.c

@@ -2948,7 +2948,7 @@ static bool gfar_add_rx_frag(struct gfar_rx_buff *rxb, u32 lstatus,
 	}
 
 	/* try reuse page */
-	if (unlikely(page_count(page) != 1))
+	if (unlikely(page_count(page) != 1 || page_is_pfmemalloc(page)))
 		return false;
 
 	/* change offset to the other half */