PRODKERNEL: optimize diff processing

During kpatch-build's orig-vs-patched comparison pass:

- ignore unchanged objfiles, which may have been rebuilt, but built
  identically. Perhaps whitespace change, or an included file changed,
  but in ways that do not affect the code generated in _this_ object

- when $SYMTAB is still current, do not recreate

- create symtabs in kpatch temp dirs, not kernel tree
  As they're not .gitignore'd, they would create worrysome clutter

- allow symtab creation in missing directories, with mkdir -p

These avoid needless kpatch failures & slowdowns, where a change has
implications outside diff footprint, like *.h that cause many files
to rebuild, without change to objects, and in parts of the tree far
removed from the source files actually patched

Tested: by t314, which changes ipv6.h, causing hundreds of modules to
	rebuild, with no actual code change

Upstream-Plan: 190514358
Signed-off-by: Pete Swain <[email protected]>

kpatch-build/kpatch-build

@@ -1135,7 +1135,10 @@ for i in $FILES; do
 	find_kobj "$i"
 	cd "$TEMPDIR" || die
 	if [[ -e "orig/$i" ]]; then
-		if [[ "$(basename "$KOBJFILE")" = vmlinux ]]; then
+		if cmp -s "orig/$i" "patched/$i"; then
+			: $i unchanged
+			continue
+		elif [[ "$(basename "$KOBJFILE")" = vmlinux ]]; then
 			KOBJFILE_NAME=vmlinux
 			KOBJFILE_PATH="$VMLINUX"
 			SYMTAB="${TEMPDIR}/${KOBJFILE_NAME}.symtab"
@@ -1149,14 +1152,17 @@ for i in $FILES; do
 		else
 			KOBJFILE_NAME=$(basename "${KOBJFILE%.ko}")
 			KOBJFILE_NAME="${KOBJFILE_NAME//-/_}"
-			KOBJFILE_PATH="${TEMPDIR}/module/$KOBJFILE"
-			SYMTAB="${KOBJFILE_PATH}.symtab"
+			KOBJFILE_PATH="${BUILDDIR}/$KOBJFILE"
+			SYMTAB="${TEMPDIR}/module/$KOBJFILE".symtab
 			SYMVERS_FILE="$BUILDDIR/Module.symvers"
 		fi
 
-		"$READELF" -s --wide "$KOBJFILE_PATH" > "$SYMTAB"
-		if [[ "$ARCH" = "ppc64le" ]]; then
-			sed -ri 's/\s+\[<localentry>: 8\]//' "$SYMTAB"
+		if ! [[ "$SYMTAB" -nt "$KOBJFILE_PATH" ]]; then
+			mkdir -p "${SYMTAB%/*}"
+			"$READELF" -s --wide "$KOBJFILE_PATH" > "$SYMTAB"
+			if [[ "$ARCH" = "ppc64le" ]]; then
+				sed -ri 's/\s+\[<localentry>: 8\]//' "$SYMTAB"
+			fi
 		fi
 
 		# create-diff-object orig.o patched.o parent-name parent-symtab