kernel: bump 4.19 to 4.19.51

Refreshed all patches.

Altered patches:
- 370-netfilter-nf_flow_table-fix-offloaded-connection-tim.patch
- 220-optimize_inlining.patch
- 640-netfilter-nf_flow_table-add-hardware-offload-support.patch

This patch also restores the initial implementation
of the ath79 perfcount IRQ issue. (78ee6b1a40)
It was wrongfully backported upstream initially and got reverted now.

Compile-tested on: cns3xxx, imx6
Runtime-tested on: cns3xxx, imx6

Signed-off-by: Koen Vandeputte <koen.vandeputte@ncentric.com>
Signed-off-by: Petr Štetiar <ynezz@true.cz>
Signed-off-by: Kevin Darbyshire-Bryant <ldir@darbyshire-bryant.me.uk>

target/linux/generic/hack-4.19/647-netfilter-flow-acct.patch

@@ -52,7 +52,7 @@
  /* For layer 4 checksum field offset. */
  #include <linux/tcp.h>
  #include <linux/udp.h>
-@@ -265,6 +266,7 @@ nf_flow_offload_ip_hook(void *priv, stru
+@@ -268,6 +269,7 @@ nf_flow_offload_ip_hook(void *priv, stru
  	skb->dev = outdev;
  	nexthop = rt_nexthop(rt, flow->tuplehash[!dir].tuple.src_v4.s_addr);
  	skb_dst_set_noref(skb, &rt->dst);
@@ -60,7 +60,7 @@
  	neigh_xmit(NEIGH_ARP_TABLE, outdev, &nexthop, skb);
  
  	return NF_STOLEN;
-@@ -482,6 +484,7 @@ nf_flow_offload_ipv6_hook(void *priv, st
+@@ -488,6 +490,7 @@ nf_flow_offload_ipv6_hook(void *priv, st
  	skb->dev = outdev;
  	nexthop = rt6_nexthop(rt, &flow->tuplehash[!dir].tuple.src_v6);
  	skb_dst_set_noref(skb, &rt->dst);