bpf: add helper to invalidate hash

Add a small helper that complements 36bbef52c7 ("bpf: direct packet
write and access for helpers for clsact progs") for invalidating the
current skb->hash after mangling on headers via direct packet write.

Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/uapi/linux/bpf.h

@@ -419,6 +419,13 @@ enum bpf_func_id {
 	 */
 	BPF_FUNC_csum_update,
 
+	/**
+	 * bpf_set_hash_invalid(skb)
+	 * Invalidate current skb>hash.
+	 * @skb: pointer to skb
+	 */
+	BPF_FUNC_set_hash_invalid,
+
 	__BPF_FUNC_MAX_ID,
 };