ebpf: add skb->priority to offset map for usage in {cls, act}_bpf

This adds the ability to read out the skb->priority from an eBPF
program, so that it can be taken into account from a tc filter
or action for the use-case where the priority is not being used
to directly override the filter classification in a qdisc, but
to tag traffic otherwise for the classifier; the priority can be
assigned from various places incl. user space, in future we may
also mangle it from an eBPF program.

Signed-off-by: Daniel Borkmann <[e-mail]>
Cc: Alexei Starovoitov <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

include/uapi/linux/bpf.h

···
 	__u32 vlan_present;
 	__u32 vlan_tci;
 	__u32 vlan_proto;
+	__u32 priority;
 };
 
 #endif /* _UAPI__LINUX_BPF_H__ */

net/core/filter.c

···
 				      offsetof(struct sk_buff, vlan_proto));
 		break;
 
+	case offsetof(struct __sk_buff, priority):
+		BUILD_BUG_ON(FIELD_SIZEOF(struct sk_buff, priority) != 4);
+
+		*insn++ = BPF_LDX_MEM(BPF_W, dst_reg, src_reg,
+				      offsetof(struct sk_buff, priority));
+		break;
+
 	case offsetof(struct __sk_buff, mark):
 		return convert_skb_access(SKF_AD_MARK, dst_reg, src_reg, insn);