bpf: Remember BTF of inner maps.

BTF is required for 'struct bpf_timer' to be recognized inside map value.
The bpf timers are supported inside inner maps.
Remember 'struct btf *' in inner_map_meta to make it available
to the verifier in the sequence:

struct bpf_map *inner_map = bpf_map_lookup_elem(&outer_map, ...);
if (inner_map)
    timer = bpf_map_lookup_elem(&inner_map, ...);

Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Yonghong Song <yhs@fb.com>
Acked-by: Martin KaFai Lau <kafai@fb.com>
Acked-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Toke Høiland-Jørgensen <toke@redhat.com>
Link: https://lore.kernel.org/bpf/20210715005417.78572-7-alexei.starovoitov@gmail.com

kernel/bpf/map_in_map.c

@@ -3,6 +3,7 @@
  */
 #include <linux/slab.h>
 #include <linux/bpf.h>
+#include <linux/btf.h>
 
 #include "map_in_map.h"
 
@@ -51,6 +52,10 @@ struct bpf_map *bpf_map_meta_alloc(int inner_map_ufd)
 	inner_map_meta->max_entries = inner_map->max_entries;
 	inner_map_meta->spin_lock_off = inner_map->spin_lock_off;
 	inner_map_meta->timer_off = inner_map->timer_off;
+	if (inner_map->btf) {
+		btf_get(inner_map->btf);
+		inner_map_meta->btf = inner_map->btf;
+	}
 
 	/* Misc members not needed in bpf_map_meta_equal() check. */
 	inner_map_meta->ops = inner_map->ops;
@@ -66,6 +71,7 @@ struct bpf_map *bpf_map_meta_alloc(int inner_map_ufd)
 
 void bpf_map_meta_free(struct bpf_map *map_meta)
 {
+	btf_put(map_meta->btf);
 	kfree(map_meta);
 }