scripts/nsdeps: support nsdeps for external module builds

scripts/nsdeps is written to take care of only in-tree modules.
Perhaps, this is not a bug, but just a design. At least,
Documentation/core-api/symbol-namespaces.rst focuses on in-tree modules.

Having said that, some people already tried nsdeps for external modules.
So, it would be nice to support it.

Reported-by: Steve French <[e-mail]>
Reported-by: Jessica Yu <[e-mail]>
Signed-off-by: Masahiro Yamada <[e-mail]>
Tested-by: Jessica Yu <[e-mail]>
Acked-by: Jessica Yu <[e-mail]>
Reviewed-by: Matthias Maennich <[e-mail]>
Tested-by: Matthias Maennich <[e-mail]>

Documentation/core-api/symbol-namespaces.rst

···
 	- notice the warning of modpost telling about a missing import
 	- run `make nsdeps` to add the import to the correct code location
 
+You can also run nsdeps for external module builds. A typical usage is::
+
+	$ make -C <path_to_kernel_src> M=$PWD nsdeps

Makefile

···
 PHONY += prepare0
 
 export MODORDER := $(extmod-prefix)modules.order
+export MODULES_NSDEPS := $(extmod-prefix)modules.nsdeps
 
 ifeq ($(KBUILD_EXTMOD),)
 core-y		+= kernel/ certs/ mm/ fs/ ipc/ security/ crypto/ block/
···
 	$(call cmd,depmod)
 
 clean-dirs := $(KBUILD_EXTMOD)
-clean: rm-files := $(KBUILD_EXTMOD)/Module.symvers
+clean: rm-files := $(KBUILD_EXTMOD)/Module.symvers $(KBUILD_EXTMOD)/modules.nsdeps
 
 PHONY += /
 /:

scripts/Makefile.modpost

···
 else
 
 MODPOST += $(subst -i,-n,$(filter -i,$(MAKEFLAGS))) -s -T - \
-	$(if $(KBUILD_NSDEPS),-d modules.nsdeps)
+	$(if $(KBUILD_NSDEPS),-d $(MODULES_NSDEPS))
 
 ifeq ($(KBUILD_EXTMOD),)
 MODPOST += $(wildcard vmlinux)

scripts/nsdeps

···
 	exit 1
 fi
 
+if [ "$KBUILD_EXTMOD" ]; then
+	src_prefix=
+else
+	src_prefix=$srctree/
+fi
+
 generate_deps_for_ns() {
 	$SPATCH --very-quiet --in-place --sp-file \
 		$srctree/scripts/coccinelle/misc/add_namespace.cocci -D ns=$1 $2
···
 	local namespaces="$*"
 	local mod_source_files="`cat $mod.mod | sed -n 1p                      \
 					      | sed -e 's/\.o/\.c/g'           \
-					      | sed "s|[^ ]* *|${srctree}/&|g"`"
+					      | sed "s|[^ ]* *|${src_prefix}&|g"`"
 	for ns in $namespaces; do
 		echo "Adding namespace $ns to module $mod.ko."
 		generate_deps_for_ns $ns "$mod_source_files"
···
 while read line
 do
 	generate_deps $line
-done < modules.nsdeps
+done < $MODULES_NSDEPS