[PATCH] samples: bpf: Add valid info for VMLINUX_BTF

[Tao Chen <chen.dylane@gmail.com>]

When i use the command 'make M=samples/bpf' to compile samples/bpf code
in ubuntu 22.04, the error info occured:
Cannot find a vmlinux for VMLINUX_BTF at any of "  /home/ubuntu/code/linux/vmlinux",
build the kernel or set VMLINUX_BTF or VMLINUX_H variable

Others often encounter this kind of issue, new kernel has the vmlinux, so we can
set the path in error info which seems more intuitive, like:
Cannot find a vmlinux for VMLINUX_BTF at any of "  /home/ubuntu/code/linux/vmlinux",
buiild the kernel or set VMLINUX_BTF like "VMLINUX_BTF=/sys/kernel/btf/vmlinux" or
VMLINUX_H variable

Signed-off-by: Tao Chen <chen.dylane@gmail.com>
---
 samples/bpf/Makefile | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/samples/bpf/Makefile b/samples/bpf/Makefile
index 4ccf4236031c..6fbe9345eb6a 100644
--- a/samples/bpf/Makefile
+++ b/samples/bpf/Makefile
@@ -326,7 +326,7 @@ $(obj)/vmlinux.h: $(VMLINUX_BTF) $(BPFTOOL)
 ifeq ($(VMLINUX_H),)
 ifeq ($(VMLINUX_BTF),)
 	$(error Cannot find a vmlinux for VMLINUX_BTF at any of "$(VMLINUX_BTF_PATHS)",\
-		build the kernel or set VMLINUX_BTF or VMLINUX_H variable)
+		build the kernel or set VMLINUX_BTF like "VMLINUX_BTF=/sys/kernel/btf/vmlinux" or VMLINUX_H variable)
 endif
 	$(Q)$(BPFTOOL) btf dump file $(VMLINUX_BTF) format c > $@
 else
-- 
2.34.1