kbuild: Remove stale asm-generic wrappers

When a header file is removed from generic-y (often accompanied by the
addition of an arch specific header), the generated wrapper file will
persist, and in some cases may still take precedence over the new arch
header.

For example commit f1fe2d21f4 ("MIPS: Add definitions for extended
context") removed ucontext.h from generic-y in arch/mips/include/asm/,
and added an arch/mips/include/uapi/asm/ucontext.h. The continued use of
the wrapper when reusing a dirty build tree resulted in build failures
in arch/mips/kernel/signal.c:

arch/mips/kernel/signal.c: In function ‘sc_to_extcontext’:
arch/mips/kernel/signal.c:142:12: error: ‘struct ucontext’ has no member named ‘uc_extcontext’
  return &uc->uc_extcontext;
            ^
Fix by detecting and removing wrapper headers in generated header
directories that do not correspond to a filename in generic-y, genhdr-y,
or the newly introduced generated-y.

Reported-by: Jacek Anaszewski <j.anaszewski@samsung.com>
Reported-by: Hauke Mehrtens <hauke@hauke-m.de>
Reported-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: James Hogan <james.hogan@imgtec.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Cc: linux-arch@vger.kernel.org
Cc: linux-mips@linux-mips.org
Cc: Paul Burton <paul.burton@imgtec.com>
Cc: linux-kbuild@vger.kernel.org
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: Michal Marek <mmarek@suse.com>
Link: http://lkml.kernel.org/r/1466808144-23209-3-git-send-email-james.hogan@imgtec.com
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

scripts/Makefile.asm-generic

@@ -13,11 +13,26 @@ include scripts/Kbuild.include
 # Create output directory if not already present
 _dummy := $(shell [ -d $(obj) ] || mkdir -p $(obj))
 
+# Stale wrappers when the corresponding files are removed from generic-y
+# need removing.
+generated-y   := $(generic-y) $(genhdr-y) $(generated-y)
+all-files     := $(patsubst %, $(obj)/%, $(generated-y))
+old-headers   := $(wildcard $(obj)/*.h)
+unwanted      := $(filter-out $(all-files),$(old-headers))
+
 quiet_cmd_wrap = WRAP    $@
 cmd_wrap = echo "\#include <asm-generic/$*.h>" >$@
 
-all: $(patsubst %, $(obj)/%, $(generic-y))
+quiet_cmd_remove = REMOVE  $(unwanted)
+cmd_remove = rm -f $(unwanted)
+
+all: $(patsubst %, $(obj)/%, $(generic-y)) FORCE
+	$(if $(unwanted),$(call cmd,remove),)
 	@:
 
 $(obj)/%.h:
 	$(call cmd,wrap)
+
+PHONY += FORCE
+.PHONY: $(PHONY)
+FORCE: ;