drivers: net: xgene: fix: Out of order descriptor bytes read

This patch fixes the following kernel crash,

	WARNING: CPU: 2 PID: 0 at net/ipv4/tcp_input.c:3079 tcp_clean_rtx_queue+0x658/0x80c()
	Call trace:
	[<fffffe0000096b7c>] dump_backtrace+0x0/0x184
	[<fffffe0000096d10>] show_stack+0x10/0x1c
	[<fffffe0000685ea0>] dump_stack+0x74/0x98
	[<fffffe00000b44e0>] warn_slowpath_common+0x88/0xb0
	[<fffffe00000b461c>] warn_slowpath_null+0x14/0x20
	[<fffffe00005b5c1c>] tcp_clean_rtx_queue+0x654/0x80c
	[<fffffe00005b6228>] tcp_ack+0x454/0x688
	[<fffffe00005b6ca8>] tcp_rcv_established+0x4a4/0x62c
	[<fffffe00005bf4b4>] tcp_v4_do_rcv+0x16c/0x350
	[<fffffe00005c225c>] tcp_v4_rcv+0x8e8/0x904
	[<fffffe000059d470>] ip_local_deliver_finish+0x100/0x26c
	[<fffffe000059dad8>] ip_local_deliver+0xac/0xc4
	[<fffffe000059d6c4>] ip_rcv_finish+0xe8/0x328
	[<fffffe000059dd3c>] ip_rcv+0x24c/0x38c
	[<fffffe0000563950>] __netif_receive_skb_core+0x29c/0x7c8
	[<fffffe0000563ea4>] __netif_receive_skb+0x28/0x7c
	[<fffffe0000563f54>] netif_receive_skb_internal+0x5c/0xe0
	[<fffffe0000564810>] napi_gro_receive+0xb4/0x110
	[<fffffe0000482a2c>] xgene_enet_process_ring+0x144/0x338
	[<fffffe0000482d18>] xgene_enet_napi+0x1c/0x50
	[<fffffe0000565454>] net_rx_action+0x154/0x228
	[<fffffe00000b804c>] __do_softirq+0x110/0x28c
	[<fffffe00000b8424>] irq_exit+0x8c/0xc0
	[<fffffe0000093898>] handle_IRQ+0x44/0xa8
	[<fffffe000009032c>] gic_handle_irq+0x38/0x7c
	[...]

Software writes poison data into the descriptor bytes[15:8] and upon
receiving the interrupt, if those bytes are overwritten by the hardware with
the valid data, software also reads bytes[7:0] and executes receive/tx
completion logic.

If the CPU executes the above two reads in out of order fashion, then the
bytes[7:0] will have older data and causing the kernel panic.  We have to
force the order of the reads and thus this patch introduces read memory
barrier between these reads.

Signed-off-by: Iyappan Subramanian <isubramanian@apm.com>
Signed-off-by: Keyur Chudgar <kchudgar@apm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>