net: Introduce netns_bpf for BPF programs attached to netns

In order to:

 (1) attach more than one BPF program type to netns, or
 (2) support attaching BPF programs to netns with bpf_link, or
 (3) support multi-prog attach points for netns

we will need to keep more state per netns than a single pointer like we
have now for BPF flow dissector program.

Prepare for the above by extracting netns_bpf that is part of struct net,
for storing all state related to BPF programs attached to netns.

Turn flow dissector callbacks for querying/attaching/detaching a program
into generic ones that operate on netns_bpf. Next patch will move the
generic callbacks into their own module.

This is similar to how it is organized for cgroup with cgroup_bpf.

Signed-off-by: Jakub Sitnicki <jakub@cloudflare.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Cc: Stanislav Fomichev <sdf@google.com>
Link: https://lore.kernel.org/bpf/20200531082846.2117903-3-jakub@cloudflare.com

include/net/net_namespace.h

@@ -33,6 +33,7 @@
 #include <net/netns/mpls.h>
 #include <net/netns/can.h>
 #include <net/netns/xdp.h>
+#include <net/netns/bpf.h>
 #include <linux/ns_common.h>
 #include <linux/idr.h>
 #include <linux/skbuff.h>
@@ -162,7 +163,8 @@ struct net {
 #endif
 	struct net_generic __rcu	*gen;
 
-	struct bpf_prog __rcu	*flow_dissector_prog;
+	/* Used to store attached BPF programs */
+	struct netns_bpf	bpf;
 
 	/* Note : following structs are cache line aligned */
 #ifdef CONFIG_XFRM