x86/callthunks: Correct calculation of dest address in is_callthunk()

GCC didn't warn on the invalid use of relocation destination
pointer, so the calculated destination value was applied to
the uninitialized pointer location in error.

Fixes: 17bce3b ("x86/callthunks: Handle %rip-relative relocations in call thunk template")
Reported-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Uros Bizjak <ubizjak@gmail.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Closes: https://lore.kernel.org/lkml/20231201035457.GA321497@dev-arch.thelio-3990X/
Link: https://lore.kernel.org/r/20231201085727.3647051-1-ubizjak@gmail.com

Author: ubizjak

arch/x86/kernel/callthunks.c

@@ -312,7 +312,7 @@ static bool is_callthunk(void *addr)
 	if (!thunks_initialized || skip_addr((void *)dest))
 		return false;
 
-	*pad = dest - tmpl_size;
+	pad = (void *)(dest - tmpl_size);
 
 	memcpy(insn_buff, skl_call_thunk_template, tmpl_size);
 	apply_relocation(insn_buff, tmpl_size, pad,