Defining SO_ATTACH_BPF and SO_DETACH_BPF to fix go build and go install on RHEL7 OS.

[fanfanbj]

Fix go build failure issue on RHEL7 OS. The error message shows as below:

$ go build -a ./elf/
github.com/iovisor/gobpf/elf
elf/module.go: In function 'bpf_attach_socket':
elf/module.go:96:38: error: 'SO_ATTACH_BPF' undeclared (first use in this function)
return setsockopt(sock, SOL_SOCKET, SO_ATTACH_BPF, &fd, sizeof(fd));
^
elf/module.go:96:38: note: each undeclared identifier is reported only once for each function it appears in
elf/module.go: In function 'bpf_detach_socket':
elf/module.go:101:38: error: 'SO_DETACH_BPF' undeclared (first use in this function)
return setsockopt(sock, SOL_SOCKET, SO_DETACH_BPF, &fd, sizeof(fd));