bpf: btf: avoid -Wreturn-type warning

gcc warns about a noreturn function possibly returning in
some configurations:

kernel/bpf/btf.c: In function 'env_type_is_resolve_sink':
kernel/bpf/btf.c:729:1: error: control reaches end of non-void function [-Werror=return-type]

Using BUG() instead of BUG_ON() avoids that warning and otherwise
does the exact same thing.

Fixes: eb3f595d ("bpf: btf: Validate type reference")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Song Liu <songliubraving@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

kernel/bpf/btf.c

@@ -749,7 +749,7 @@ static bool env_type_is_resolve_sink(const struct btf_verifier_env *env,
 			!btf_type_is_array(next_type) &&
 			!btf_type_is_struct(next_type);
 	default:
-		BUG_ON(1);
+		BUG();
 	}
 }