Fix "make install" with DESTDIR set (#16995)

"DESTDIR=/path/to/target/root/ make install" may fail when installing to
a root that contains an existing lib/modules structure. When run as root
we may even affect the wrong kernel (the build system's one, or, if
running a different version, some other directory in /lib/modules, but
not the desired one installed in DESTDIR).

Add a missing reference to the INSTALL_MOD_PATH root when calling
"depmod" during "make install"

Also add a switch "DONT_DELETE_MODULES_FILES=1" that skips the removal
of files named "modules.*" prior to running depmod.

Signed-off-by: Christian Kohlschütter <[email protected]>
Closes #16994

Reviewed-by: Rob Norris <[email protected]>
Reviewed-by: Brian Behlendorf <[email protected]>
Reviewed-by: Tony Hutter <[email protected]>

module/Makefile.in

@@ -93,7 +93,7 @@ modules_install-Linux: modules_uninstall-Linux-legacy
 		$(if @KERNEL_ARCH@,ARCH=@KERNEL_ARCH@) \
 		KERNELRELEASE=@LINUX_VERSION@
 	@# Remove extraneous build products when packaging
-	if [ -n "$(DESTDIR)" ]; then \
+	if [ -n "$(DESTDIR)" ] && [ "$(DONT_DELETE_MODULES_FILES)" != "1" ]; then \
 		find $(KMODDIR) -name 'modules.*' -delete; \
 	fi
 	@# Debian ships tiny fake System.map files that are
@@ -106,7 +106,7 @@ modules_install-Linux: modules_uninstall-Linux-legacy
 	{ [ -f "$$sysmap" ] && [ $$(wc -l < "$$sysmap") -ge 100 ]; } || \
 		sysmap=$(INSTALL_MOD_PATH)/usr/lib/debug/boot/System.map-@LINUX_VERSION@; \
 	if [ -f $$sysmap ]; then \
-		depmod -ae -F $$sysmap @LINUX_VERSION@; \
+		depmod -ae -F $$sysmap @LINUX_VERSION@ -b $(INSTALL_MOD_PATH)/; \
 	fi
 
 modules_install-FreeBSD: