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: 17bce3b2 ("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