kbuild: make C=2 now force sparse to be run for all .c files

With make C=2 sparse ($(CHECK)) will be run on all .c files also if they
do not need to be compiled.
Usefull to run sparse on a fully compiled kernel tree.
Implemented on request from Al Viro (although he liked to be able to
run sparse without building any source).
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>

Makefile

@@ -958,7 +958,8 @@ help:
 
 	@echo  '  make V=0|1 [targets] 0 => quiet build (default), 1 => verbose build'
 	@echo  '  make O=dir [targets] Locate all output files in "dir", including .config'
-	@echo  '  make C=1   [targets] Check all c source with checker tool'
+	@echo  '  make C=1   [targets] Check all c source with $$CHECK (sparse)'
+	@echo  '  make C=2   [targets] Force check of all c source with $$CHECK (sparse)'
 	@echo  ''
 	@echo  'Execute "make" or "make all" to build all targets marked with [*] '
 	@echo  'For further info see the ./README file'

scripts/Makefile.build

@@ -83,8 +83,13 @@ __build: $(if $(KBUILD_BUILTIN),$(builtin-target) $(lib-target) $(extra-y)) \
 
 # Linus' kernel sanity checking tool
 ifneq ($(KBUILD_CHECKSRC),0)
-quiet_cmd_checksrc = CHECK   $<
+  ifeq ($(KBUILD_CHECKSRC),2)
+    quiet_cmd_force_checksrc = CHECK   $<
+          cmd_force_checksrc = $(CHECK) $(c_flags) $< ;
+  else
+      quiet_cmd_checksrc     = CHECK   $<
             cmd_checksrc     = $(CHECK) $(c_flags) $< ;
+  endif
 endif
 
 
@@ -182,11 +187,13 @@ endef
 # Built-in and composite module parts
 
 %.o: %.c FORCE
+	$(call cmd,force_checksrc)
 	$(call if_changed_rule,cc_o_c)
 
 # Single-part modules are special since we need to mark them in $(MODVERDIR)
 
 $(single-used-m): %.o: %.c FORCE
+	$(call cmd,force_checksrc)
 	$(call if_changed_rule,cc_o_c)
 	@{ echo $(@:.o=.ko); echo $@; } > $(MODVERDIR)/$(@F:.o=.mod)