x86/alternatives: Simplify smp_text_poke_single() by using tp_vec and existing APIs

Instead of constructing a vector on-stack, just use the already
available batch-patching vector - which should always be empty
at this point.

This will allow subsequent simplifications.

Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: Juergen Gross <jgross@suse.com>
Cc: "H . Peter Anvin" <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: https://lore.kernel.org/r/20250411054105.2341982-25-mingo@kernel.org