selftests/bpf: fix compile errors due to unsync linux/in6.h and netinet/in.h

I meet below compile errors:
"
In file included from test_tcpnotify_kern.c:12:
/usr/include/netinet/in.h:101:5: error: expected identifier
    IPPROTO_HOPOPTS = 0,   /* IPv6 Hop-by-Hop options.  */
    ^
/usr/include/linux/in6.h:131:26: note: expanded from macro 'IPPROTO_HOPOPTS'
                                ^
In file included from test_tcpnotify_kern.c:12:
/usr/include/netinet/in.h:103:5: error: expected identifier
    IPPROTO_ROUTING = 43,  /* IPv6 routing header.  */
    ^
/usr/include/linux/in6.h:132:26: note: expanded from macro 'IPPROTO_ROUTING'
                                ^
In file included from test_tcpnotify_kern.c:12:
/usr/include/netinet/in.h:105:5: error: expected identifier
    IPPROTO_FRAGMENT = 44, /* IPv6 fragmentation header.  */
    ^
/usr/include/linux/in6.h:133:26: note: expanded from macro 'IPPROTO_FRAGMENT'
"
The same compile errors are reported for test_tcpbpf_kern.c too.

My environment:
lsb_release -a:
No LSB modules are available.
Distributor ID: Ubuntu
Description:    Ubuntu 16.04.6 LTS
Release:        16.04
Codename:       xenial

dpkg -l | grep libc-dev:
ii  libc-dev-bin              2.23-0ubuntu11           amd64        GNU C Library: Development binaries
ii  linux-libc-dev:amd64      4.4.0-145.171            amd64        Linux Kernel Headers for development.

The reason is linux/in6.h and netinet/in.h aren't synchronous about how to
handle the same definitions, IPPROTO_HOPOPTS, etc.

This patch fixes the compile errors by moving <netinet/in.h> to before the
<linux/*.h>.

Signed-off-by: Wang YanQing <udknight@gmail.com>
Acked-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>