samples/bpf: Remove compiler warnings

Commit 5fbc220862 ("tools/libpf: Add offsetof/container_of macro
in bpf_helpers.h") added macros offsetof/container_of to
bpf_helpers.h. Unfortunately, it caused compilation warnings
below for a few samples/bpf programs:
  In file included from /data/users/yhs/work/net-next/samples/bpf/sockex2_kern.c:4:
  In file included from /data/users/yhs/work/net-next/include/uapi/linux/in.h:24:
  In file included from /data/users/yhs/work/net-next/include/linux/socket.h:8:
  In file included from /data/users/yhs/work/net-next/include/linux/uio.h:8:
  /data/users/yhs/work/net-next/include/linux/kernel.h:992:9: warning: 'container_of' macro redefined [-Wmacro-redefined]
          ^
  /data/users/yhs/work/net-next/tools/lib/bpf/bpf_helpers.h:46:9: note: previous definition is here
          ^
  1 warning generated.
    CLANG-bpf  samples/bpf/sockex3_kern.o

In all these cases, bpf_helpers.h is included first, followed by other
standard headers. The macro container_of is defined unconditionally
in kernel.h, causing the compiler warning.

The fix is to move bpf_helpers.h after standard headers.

Signed-off-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Andrii Nakryiko <andriin@fb.com>
Link: https://lore.kernel.org/bpf/20200513180223.2949987-1-yhs@fb.com

samples/bpf/offwaketime_kern.c

@@ -5,12 +5,12 @@
  * License as published by the Free Software Foundation.
  */
 #include <uapi/linux/bpf.h>
-#include <bpf/bpf_helpers.h>
-#include <bpf/bpf_tracing.h>
 #include <uapi/linux/ptrace.h>
 #include <uapi/linux/perf_event.h>
 #include <linux/version.h>
 #include <linux/sched.h>
+#include <bpf/bpf_helpers.h>
+#include <bpf/bpf_tracing.h>
 
 #define _(P) ({typeof(P) val; bpf_probe_read(&val, sizeof(val), &P); val;})
 

samples/bpf/sockex2_kern.c

@@ -1,12 +1,12 @@
 #include <uapi/linux/bpf.h>
-#include <bpf/bpf_helpers.h>
-#include "bpf_legacy.h"
 #include <uapi/linux/in.h>
 #include <uapi/linux/if.h>
 #include <uapi/linux/if_ether.h>
 #include <uapi/linux/ip.h>
 #include <uapi/linux/ipv6.h>
 #include <uapi/linux/if_tunnel.h>
+#include <bpf/bpf_helpers.h>
+#include "bpf_legacy.h"
 #define IP_MF		0x2000
 #define IP_OFFSET	0x1FFF
 

samples/bpf/sockex3_kern.c

@@ -5,8 +5,6 @@
  * License as published by the Free Software Foundation.
  */
 #include <uapi/linux/bpf.h>
-#include <bpf/bpf_helpers.h>
-#include "bpf_legacy.h"
 #include <uapi/linux/in.h>
 #include <uapi/linux/if.h>
 #include <uapi/linux/if_ether.h>
@@ -14,6 +12,8 @@
 #include <uapi/linux/ipv6.h>
 #include <uapi/linux/if_tunnel.h>
 #include <uapi/linux/mpls.h>
+#include <bpf/bpf_helpers.h>
+#include "bpf_legacy.h"
 #define IP_MF		0x2000
 #define IP_OFFSET	0x1FFF