kbuild: enable kernel-doc -Wall for W=2

For W=2, we can enable more kernel-doc warnings,
such as missing return value descriptions etc.
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>

scripts/Makefile.build

@@ -101,7 +101,9 @@ else ifeq ($(KBUILD_CHECKSRC),2)
 endif
 
 ifneq ($(KBUILD_EXTRA_WARN),)
-  cmd_checkdoc = $(srctree)/scripts/kernel-doc -none $(KDOCFLAGS) $<
+  cmd_checkdoc = $(srctree)/scripts/kernel-doc -none $(KDOCFLAGS) \
+        $(if $(findstring 2, $(KBUILD_EXTRA_WARN)), -Wall) \
+        $<
 endif
 
 # Compile C sources (.c)