bpf: Add a verbose message if map limit is reached

When more than 64 maps are used by a program and its subprograms the
verifier returns -E2BIG. Add a verbose message which highlights the
source of the error and also print the actual limit.
Signed-off-by: Anton Protopopov <aspsk@isovalent.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Yonghong Song <yonghong.song@linux.dev>
Acked-by: John Fastabend <john.fastabend@gmail.com>
Link: https://lore.kernel.org/bpf/20240402073347.195920-1-aspsk@isovalent.com

kernel/bpf/verifier.c

@@ -18348,6 +18348,8 @@ static int resolve_pseudo_ldimm64(struct bpf_verifier_env *env)
 			}
 
 			if (env->used_map_cnt >= MAX_USED_MAPS) {
+				verbose(env, "The total number of maps per program has reached the limit of %u\n",
+					MAX_USED_MAPS);
 				fdput(f);
 				return -E2BIG;
 			}