x86/alternatives: Use a temporary buffer when optimizing NOPs

Instead of optimizing NOPs in-place, use a temporary buffer like the
usual alternatives patching flow does. This obviates the need to grab
locks when patching, see

  67789775

 ("x86/alternatives: Disable interrupts and sync when optimizing NOPs in place")

While at it, add nomenclature definitions clarifying and simplifying the
naming of function-local variables in the alternatives code.

Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Link: https://lore.kernel.org/r/20240130105941.19707-2-bp@alien8.de