x86/alternatives: Add text_mutex) assert to smp_text_poke_batch_flush()

It's possible to escape the text_mutex-held assert in
smp_text_poke_batch_process() if the caller uses a properly
batched and sorted series of patch requests, so add
an explicit lockdep_assert_held() to make sure it's
held by all callers.

All text_poke_int3_*() APIs will call either smp_text_poke_batch_process()
or smp_text_poke_batch_flush() internally.

The text_mutex must be held, because tp_vec and tp_vec_nr et al
are all globals, and the INT3 patching machinery itself relies on
external serialization.

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/20250411054105.2341982-22-mingo@kernel.org