selftests: net: Fix O=dir builds

The BPF Makefile in net/bpf did incorrect path substitution for O=dir
builds, e.g.

  make O=/tmp/kselftest headers
  make O=/tmp/kselftest -C tools/testing/selftests

would fail in selftest builds [1] net/ with

  clang-16: error: no such file or directory: 'kselftest/net/bpf/nat6to4.c'
  clang-16: error: no input files

Add a pattern prerequisite and an order-only-prerequisite (for
creating the directory), to resolve the issue.

[1] https://lore.kernel.org/all/202212060009.34CkQmCN-lkp@intel.com/

Reported-by: kernel test robot <lkp@intel.com>
Fixes: 837a3d6 ("selftests: net: Add cross-compilation support for BPF programs")
Signed-off-by: Björn Töpel <bjorn@rivosinc.com>
Link: https://lore.kernel.org/r/20221206102838.272584-1-bjorn@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

tools/testing/selftests/net/bpf/Makefile

@@ -12,7 +12,7 @@ CCINCLUDE += -I$(SCRATCH_DIR)/include
 
 BPFOBJ := $(BUILD_DIR)/libbpf/libbpf.a
 
-MAKE_DIRS := $(BUILD_DIR)/libbpf
+MAKE_DIRS := $(BUILD_DIR)/libbpf $(OUTPUT)/bpf
 $(MAKE_DIRS):
 	mkdir -p $@
 
@@ -37,8 +37,8 @@ endif
 
 CLANG_SYS_INCLUDES = $(call get_sys_includes,$(CLANG),$(CLANG_TARGET_ARCH))
 
-$(TEST_CUSTOM_PROGS): $(BPFOBJ)
-	$(CLANG) -O2 -target bpf -c $(@:.o=.c) $(CCINCLUDE) $(CLANG_SYS_INCLUDES) -o $@
+$(TEST_CUSTOM_PROGS): $(OUTPUT)/%.o: %.c $(BPFOBJ) | $(MAKE_DIRS)
+	$(CLANG) -O2 -target bpf -c $< $(CCINCLUDE) $(CLANG_SYS_INCLUDES) -o $@
 
 $(BPFOBJ): $(wildcard $(BPFDIR)/*.[ch] $(BPFDIR)/Makefile)		       \
 	   $(APIDIR)/linux/bpf.h					       \