kbuild: Add a bug trap for people playing with SUBDIRS too much

If SUBDIRS is set manually on the command line, the contents of
.tmp_versions are not deleted before descending and can accumulate
stale entries. Print a warning if that case is detected, but deal with
it gracefully.

scripts/Makefile.modver

@@ -9,7 +9,12 @@ include scripts/Makefile.lib
 
 #
 
-modules := $(shell cd $(MODVERDIR); cat *.mod)
+__modules := $(shell cd $(MODVERDIR); cat *.mod)
+modules := $(patsubst %.o,%.ko,$(wildcard $(__modules:.ko=.o)))
+
+ifneq ($(filter-out $(modules),$(__modules)),)
+  $(warning *** Uh-oh, you have stale module entries. You messed with SUBDIRS, don't complain if something goes wrong.)
+endif
 
 __modversions: $(modules)
 	@: