samples/bpf: fix a build issue

With latest net-next:

====
clang  -nostdinc -isystem /usr/lib/gcc/x86_64-redhat-linux/6.3.1/include -I./arch/x86/include -I./arch/x86/include/generated/uapi -I./arch/x86/include/generated  -I./include -I./arch/x86/include/uapi -I./include/uapi -I./include/generated/uapi -include ./include/linux/kconfig.h  -Isamples/bpf \
    -D__KERNEL__ -D__ASM_SYSREG_H -Wno-unused-value -Wno-pointer-sign \
    -Wno-compare-distinct-pointer-types \
    -Wno-gnu-variable-sized-type-not-at-end \
    -Wno-address-of-packed-member -Wno-tautological-compare \
    -Wno-unknown-warning-option \
    -O2 -emit-llvm -c samples/bpf/tcp_synrto_kern.c -o -| llc -march=bpf -filetype=obj -o samples/bpf/tcp_synrto_kern.o
samples/bpf/tcp_synrto_kern.c:20:10: fatal error: 'bpf_endian.h' file not found
          ^~~~~~~~~~~~~~
1 error generated.
====

net has the same issue.

Add support for ntohl and htonl in tools/testing/selftests/bpf/bpf_endian.h.
Also move bpf_helpers.h from samples/bpf to selftests/bpf and change
compiler include logic so that programs in samples/bpf can access the headers
in selftests/bpf, but not the other way around.
Signed-off-by: Yonghong Song <yhs@fb.com>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Lawrence Brakmo <brakmo@fb.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

samples/bpf/Makefile

@@ -207,6 +207,7 @@ $(obj)/tracex5_kern.o: $(obj)/syscall_nrs.h
 # useless for BPF samples.
 $(obj)/%.o: $(src)/%.c
 	$(CLANG) $(NOSTDINC_FLAGS) $(LINUXINCLUDE) $(EXTRA_CFLAGS) -I$(obj) \
+		-I$(srctree)/tools/testing/selftests/bpf/ \
 		-D__KERNEL__ -D__ASM_SYSREG_H -Wno-unused-value -Wno-pointer-sign \
 		-Wno-compare-distinct-pointer-types \
 		-Wno-gnu-variable-sized-type-not-at-end \

tools/testing/selftests/bpf/Makefile

@@ -37,6 +37,5 @@ CLANG ?= clang
 
 %.o: %.c
 	$(CLANG) -I. -I./include/uapi -I../../../include/uapi \
-		-I../../../../samples/bpf/ \
 		-Wno-compare-distinct-pointer-types \
 		-O2 -target bpf -c $< -o $@

tools/testing/selftests/bpf/bpf_endian.h

@@ -23,11 +23,19 @@
 # define __bpf_htons(x)			__builtin_bswap16(x)
 # define __bpf_constant_ntohs(x)	___constant_swab16(x)
 # define __bpf_constant_htons(x)	___constant_swab16(x)
+# define __bpf_ntohl(x)			__builtin_bswap32(x)
+# define __bpf_htonl(x)			__builtin_bswap32(x)
+# define __bpf_constant_ntohl(x)	___constant_swab32(x)
+# define __bpf_constant_htonl(x)	___constant_swab32(x)
 #elif __BYTE_ORDER__ == __ORDER_BIG_ENDIAN__
 # define __bpf_ntohs(x)			(x)
 # define __bpf_htons(x)			(x)
 # define __bpf_constant_ntohs(x)	(x)
 # define __bpf_constant_htons(x)	(x)
+# define __bpf_ntohl(x)			(x)
+# define __bpf_htonl(x)			(x)
+# define __bpf_constant_ntohl(x)	(x)
+# define __bpf_constant_htonl(x)	(x)
 #else
 # error "Fix your compiler's __BYTE_ORDER__?!"
 #endif
@@ -38,5 +46,11 @@
 #define bpf_ntohs(x)				\
 	(__builtin_constant_p(x) ?		\
 	 __bpf_constant_ntohs(x) : __bpf_ntohs(x))
+#define bpf_htonl(x)				\
+	(__builtin_constant_p(x) ?		\
+	 __bpf_constant_htonl(x) : __bpf_htonl(x))
+#define bpf_ntohl(x)				\
+	(__builtin_constant_p(x) ?		\
+	 __bpf_constant_ntohl(x) : __bpf_ntohl(x))
 
 #endif /* __BPF_ENDIAN__ */