samples: bpf: Fix vmlinux.h generation for XDP samples

Generate vmlinux.h only from the in-tree vmlinux, and remove enum
declarations that would cause a build failure in case of version
mismatches.

There are now two options when building the samples:
1. Compile the kernel to use in-tree vmlinux for vmlinux.h
2. Override VMLINUX_BTF for samples using something like this:
   make VMLINUX_BTF=/sys/kernel/btf/vmlinux -C samples/bpf

This change was tested with relative builds, e.g. cases like:
 * make O=build -C samples/bpf
 * make KBUILD_OUTPUT=build -C samples/bpf
 * make -C samples/bpf
 * cd samples/bpf && make

When a suitable VMLINUX_BTF is not found, the following message is
printed:
/home/kkd/src/linux/samples/bpf/Makefile:333: *** Cannot find a vmlinux
for VMLINUX_BTF at any of "  ./vmlinux", build the kernel or set
VMLINUX_BTF variable.  Stop.

Fixes: 384b6b3b (samples: bpf: Add vmlinux.h generation support)
Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Toke Høiland-Jørgensen <toke@redhat.com>
Link: https://lore.kernel.org/bpf/20210928054608.1799021-1-memxor@gmail.com

samples/bpf/Makefile

@@ -322,17 +322,11 @@ $(obj)/hbm_edt_kern.o: $(src)/hbm.h $(src)/hbm_kern.h
 
 -include $(BPF_SAMPLES_PATH)/Makefile.target
 
-VMLINUX_BTF_PATHS ?= $(if $(O),$(O)/vmlinux)				\
-		     $(if $(KBUILD_OUTPUT),$(KBUILD_OUTPUT)/vmlinux)	\
-		     ../../../../vmlinux				\
-		     /sys/kernel/btf/vmlinux				\
-		     /boot/vmlinux-$(shell uname -r)
+VMLINUX_BTF_PATHS ?= $(abspath $(if $(O),$(O)/vmlinux))				\
+		     $(abspath $(if $(KBUILD_OUTPUT),$(KBUILD_OUTPUT)/vmlinux))	\
+		     $(abspath ./vmlinux)
 VMLINUX_BTF ?= $(abspath $(firstword $(wildcard $(VMLINUX_BTF_PATHS))))
 
-ifeq ($(VMLINUX_BTF),)
-$(error Cannot find a vmlinux for VMLINUX_BTF at any of "$(VMLINUX_BTF_PATHS)")
-endif
-
 $(obj)/vmlinux.h: $(VMLINUX_BTF) $(BPFTOOL)
 ifeq ($(VMLINUX_H),)
 	$(Q)$(BPFTOOL) btf dump file $(VMLINUX_BTF) format c > $@
@@ -340,6 +334,11 @@ else
 	$(Q)cp "$(VMLINUX_H)" $@
 endif
 
+ifeq ($(VMLINUX_BTF),)
+	$(error Cannot find a vmlinux for VMLINUX_BTF at any of "$(VMLINUX_BTF_PATHS)",\
+		build the kernel or set VMLINUX_BTF variable)
+endif
+
 clean-files += vmlinux.h
 
 # Get Clang's default includes on this system, as opposed to those seen by

samples/bpf/xdp_redirect_map_multi.bpf.c

@@ -5,11 +5,6 @@
 #include "xdp_sample.bpf.h"
 #include "xdp_sample_shared.h"
 
-enum {
-	BPF_F_BROADCAST		= (1ULL << 3),
-	BPF_F_EXCLUDE_INGRESS	= (1ULL << 4),
-};
-
 struct {
 	__uint(type, BPF_MAP_TYPE_DEVMAP_HASH);
 	__uint(key_size, sizeof(int));