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: eb3f595dab40 ("bpf: btf: Validate type reference")
Signed-off-by: Arnd Bergmann <[e-mail]>
Acked-by: Song Liu <[e-mail]>
Signed-off-by: Daniel Borkmann <[e-mail]>

kernel/bpf/btf.c

···
 			!btf_type_is_array(next_type) &&
 			!btf_type_is_struct(next_type);
 	default:
-		BUG_ON(1);
+		BUG();
 	}
 }