ebpf: move CONFIG_BPF_SYSCALL-only function declarations

Masami noted that it would be better to hide the remaining CONFIG_BPF_SYSCALL-only
function declarations within the BPF header ifdef, w/o else path dummy alternatives
since these functions are not supposed to have a user outside of CONFIG_BPF_SYSCALL.

Suggested-by: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
Reference: http://article.gmane.org/gmane.linux.kernel.api/8658


Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: David S. Miller <davem@davemloft.net>