ipv6:add IPV6_PKTINFO sticky option support to setsockopt()

There are three reasons for me to add this support:
1.When no interface is specified in an IPV6_PKTINFO ancillary data
item, the interface specified in an IPV6_PKTINFO sticky optionis is used.

RFC3542:
6.7. Summary of Outgoing Interface Selection

This document and [RFC-3493] specify various methods that affect the selection of the packet's outgoing interface. This subsection summarizes the ordering among those in order to ensure deterministic behavior.

For a given outgoing packet on a given socket, the outgoing interface is determined in the following order:

1. if an interface is specified in an IPV6_PKTINFO ancillary data item, the interface is used.

2. otherwise, if an interface is specified in an IPV6_PKTINFO sticky option, the interface is used.

2.When no IPV6_PKTINFO ancillary data is received,getsockopt() should
return the sticky option value which set with setsockopt().

RFC 3542:
Issuing getsockopt() for the above options will return the sticky option value i.e., the value set with setsockopt(). If no sticky option value has been set getsockopt() will return the following values:

3.Make the setsockopt implementation POSIX compliant.

---
 include/linux/ipv6.h     |    1 +
 net/ipv6/ipv6_sockglue.c |   22 ++++++++++++++++++++++
 2 files changed, 23 insertions(+), 0 deletions(-)

index 641e026..28fcdbc 100644
@@ -279,6 +279,7 @@ struct ipv6_pinfo {
 	struct in6_addr 	rcv_saddr;
 	struct in6_addr		daddr;
 	struct in6_addr		*daddr_cache;
+	struct in6_pktinfo	sticky_pktinfo;
 #ifdef CONFIG_IPV6_SUBTREES
 	struct in6_addr		*saddr_cache;
 #endif

index 4e5eac3..02f64be 100644
@@ -390,6 +390,28 @@ sticky_done:
 		break;
 	}
 
+	case IPV6_PKTINFO:
+	{
+		struct in6_pktinfo pkt;
+
+		if (optlen == 0)
+			goto e_inval;
+		else if (optlen < sizeof(struct in6_pktinfo) || optval == NULL)
+			goto e_inval;
+
+		if (copy_from_user(&pkt, optval, optlen)) {
+				retv = -EFAULT;
+				break;
+		}
+		if(sk->sk_bound_dev_if && pkt.ipi6_ifindex != sk->sk_bound_dev_if)
+			goto e_inval;
+		
+		np->sticky_pktinfo.ipi6_ifindex = pkt.ipi6_ifindex;
+		ipv6_addr_copy(&np->sticky_pktinfo.ipi6_addr, &pkt.ipi6_addr);
+		retv = 0;
+		break;
+	}
+
 	case IPV6_2292PKTOPTIONS:
 	{
 		struct ipv6_txoptions *opt = NULL;

1.5.3.8