objtool/klp: Fix bug table handling for __WARN_printf()

Running objtool klp-diff on a changed function which uses WARN() can
fail with:

vmlinux.o: error: objtool: md_run+0x866: failed to convert reloc sym '__bug_table' to its proper format

The problem is that since commit 5b472b6e5bd9 ("x86_64/bug: Implement
__WARN_printf()"), each __WARN_printf() call site now directly
references its bug table entry. klp-diff errors out when it can't
convert such section-based references to object symbols (because bug
table entries don't have symbols).

Luckily, klp-diff already has code to create symbols for bug table
entries. Move that code earlier, before function diffing.

Fixes: dd590d4d57eb ("objtool/klp: Introduce klp diff subcommand for diffing object files")
Fixes: 5b472b6e5bd9 ("x86_64/bug: Implement __WARN_printf()")
Reported-by: Song Liu <[e-mail]>
Tested-by: Song Liu <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Josh Poimboeuf <[e-mail]>

tools/objtool/klp-diff.c

···
 {
 	struct section *patched_sec;
 
-	if (create_fake_symbols(e->patched))
-		return -1;
-
 	for_each_sec(e->patched, patched_sec) {
 		if (is_special_section(patched_sec)) {
 			if (clone_special_section(e, patched_sec))
···
 
 	e.out = elf_create_file(&e.orig->ehdr, argv[2]);
 	if (!e.out)
+		return -1;
+
+	/*
+	 * Special section fake symbols are needed so that individual special
+	 * section entries can be extracted by clone_special_sections().
+	 *
+	 * Note the fake symbols are also needed by clone_included_functions()
+	 * because __WARN_printf() call sites add references to bug table
+	 * entries in the calling functions.
+	 */
+	if (create_fake_symbols(e.patched))
 		return -1;
 
 	if (clone_included_functions(&e))