Add optional check for bad kernel-doc comments

Implement a '-none' output mode for kernel-doc which will only output
warning messages, and suppresses the warning message about there being
no kernel-doc in the file.

If the build has requested additional warnings, automatically check all
.c files.  This patch does not check .h files.  Enabling the warning
by default would add about 1300 warnings, so it's default off for now.
People who care can use this to check they didn't break the docs and
maybe we'll get all the warnings fixed and be able to enable this check
by default in the future.

Signed-off-by: Matthew Wilcox <mawilcox@microsoft.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

scripts/Makefile.build

@@ -108,6 +108,10 @@ ifneq ($(KBUILD_CHECKSRC),0)
   endif
 endif
 
+ifneq ($(KBUILD_ENABLE_EXTRA_GCC_CHECKS),)
+  cmd_checkdoc = $(srctree)/scripts/kernel-doc -none $< ;
+endif
+
 # Do section mismatch analysis for each module/built-in.o
 ifdef CONFIG_DEBUG_SECTION_MISMATCH
   cmd_secanalysis = ; scripts/mod/modpost $@
@@ -289,6 +293,7 @@ define rule_cc_o_c
 	$(call echo-cmd,checksrc) $(cmd_checksrc)			  \
 	$(call cmd_and_fixdep,cc_o_c)					  \
 	$(cmd_modversions_c)						  \
+	$(cmd_checkdoc)							  \
 	$(call echo-cmd,objtool) $(cmd_objtool)				  \
 	$(call echo-cmd,record_mcount) $(cmd_record_mcount)
 endef