kbuild: readd -w option when vmlinux.o or Module.symver is missing

Commit 63ffe00d8c ("kbuild: Fix running modpost with musl libc")
accidentally turned the unresolved symbol warnings into errors when
vmlinux.o (for in-tree builds) or Module.symver (for external module
builds) is missing.

In those cases, unresolved symbols are expected, but the -w option
is not set because 'missing-input' is referenced before set.

Move $(missing-input) back to the original place. This should be fine
for musl libc because vmlinux.o and -w are not added at the same time.

With this change, -w may be passed twice, but it is not a big deal.

Link: https://lore.kernel.org/all/b56a03b8-2a2a-f833-a5d2-cdc50a7ca2bb@cschramm.eu/
Fixes: 63ffe00d8c ("kbuild: Fix running modpost with musl libc")
Reported-by: Christopher Schramm <debian@cschramm.eu>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Tested-by: Samuel Holland <samuel@sholland.org>

scripts/Makefile.modpost

@@ -44,6 +44,7 @@ modpost-args =										\
 	$(if $(CONFIG_MODVERSIONS),-m)							\
 	$(if $(CONFIG_MODULE_SRCVERSION_ALL),-a)					\
 	$(if $(CONFIG_SECTION_MISMATCH_WARN_ONLY),,-E)					\
+	$(if $(KBUILD_MODPOST_WARN),-w)							\
 	$(if $(KBUILD_NSDEPS),-d $(MODULES_NSDEPS))					\
 	$(if $(CONFIG_MODULE_ALLOW_MISSING_NAMESPACE_IMPORTS)$(KBUILD_NSDEPS),-N)	\
 	-o $@
@@ -55,10 +56,6 @@ ifneq ($(findstring i,$(filter-out --%,$(MAKEFLAGS))),)
 modpost-args += -n
 endif
 
-ifneq ($(KBUILD_MODPOST_WARN)$(missing-input),)
-modpost-args += -w
-endif
-
 # Read out modules.order to pass in modpost.
 # Otherwise, allmodconfig would fail with "Argument list too long".
 ifdef KBUILD_MODULES
@@ -124,6 +121,10 @@ modpost-args += -e $(addprefix -i , $(KBUILD_EXTRA_SYMBOLS))
 
 endif # ($(KBUILD_EXTMOD),)
 
+ifneq ($(missing-input),)
+modpost-args += -w
+endif
+
 quiet_cmd_modpost = MODPOST $@
       cmd_modpost = \
 	$(if $(missing-input), \