use dynamic prefix offset instead of hardcoding

In commit [1], kpatch added support for function padding,
and CONFIG_CFI_CLANG, which hardcoded a value of 16 for
the prefix size.

In some cases, the padding around __cfi prefixed functions can
vary. For example, in Photon OS 5.0, the __cfi prefix
size is modified in a patch for the gcc RAP plugin [2].

Since we have read the prefix size anyways, we can use it
instead of hardcoding.

Ref:
1. 3e54c63
2. https://github.com/vmware/photon/blob/5.0/SPECS/linux/secure/gcc-rap-plugin-with-kcfi.patch

Signed-off-by: Brennan Lamoreaux <[email protected]>

kpatch-build/create-diff-object.c

@@ -236,19 +236,19 @@ static struct rela *toc_rela(const struct rela *rela)
 static void kpatch_bundle_symbols(struct kpatch_elf *kelf)
 {
 	struct symbol *sym;
-	unsigned int expected_offset;
+	uint64_t expected_offset;
 
 	list_for_each_entry(sym, &kelf->symbols, list) {
 		if (is_bundleable(sym)) {
 			if (sym->pfx)
-				expected_offset = 16;
+				expected_offset = sym->pfx->sym.st_size;
 			else if (is_gcc6_localentry_bundled_sym(kelf, sym))
 				expected_offset = 8;
 			else
 				expected_offset = 0;
 
 			if (sym->sym.st_value != expected_offset) {
-				ERROR("symbol %s at offset %lu within section %s, expected %u",
+				ERROR("symbol %s at offset %lu within section %s, expected %lu",
 				      sym->name, sym->sym.st_value,
 				      sym->sec->name, expected_offset);
 			}

kpatch-build/kpatch-elf.c

@@ -470,7 +470,7 @@ static void kpatch_link_prefixed_functions(struct kpatch_elf *kelf)
 
 		list_for_each_entry(func, &kelf->symbols, list) {
 			if (func->type == STT_FUNC && func->sec == pfx->sec &&
-			    func->sym.st_value == pfx->sym.st_value + 16) {
+			    func->sym.st_value == pfx->sym.st_value + pfx->sym.st_size) {
 
 				/*
 				 * If a func has aliases, it's possible for