bpf: Add a verbose message when the BTF limit is reached

When a BPF program which is being loaded reaches the map limit
(MAX_USED_MAPS) or the BTF limit (MAX_USED_BTFS) the -E2BIG is
returned. However, in the former case there is an accompanying
verifier verbose message, and in the latter case there is not.
Add a verbose message to make the behaviour symmetrical.

Reported-by: Kevin Sheldrake <[e-mail]>
Signed-off-by: Anton Protopopov <[e-mail]>
Signed-off-by: Daniel Borkmann <[e-mail]>
Acked-by: Daniel Borkmann <[e-mail]>
Link: https://lore.kernel.org/bpf/[e-mail]

kernel/bpf/verifier.c

···
 		if (env->used_btfs[i].btf == btf)
 			return i;
 
-	if (env->used_btf_cnt >= MAX_USED_BTFS)
+	if (env->used_btf_cnt >= MAX_USED_BTFS) {
+		verbose(env, "The total number of btfs per program has reached the limit of %u\n",
+			MAX_USED_BTFS);
 		return -E2BIG;
+	}
 
 	btf_get(btf);