UPSTREAM: smp: reword smp call IPI comment

Accessing the call_single_queue hasn't involved a spinlock since 2014:

  6897fc22 ("kernel: use lockless list for smp_call_function_single")

The llist operations (namely cmpxchg() and xchg()) provide similar ordering
guarantees, update the comment to lessen confusion.

Bug: 326196720
Signed-off-by: Valentin Schneider <vschneid@redhat.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/20230307143558.294354-7-vschneid@redhat.com


(cherry picked from commit 253a0fb4)
Signed-off-by: Qais Yousef <qyousef@google.com>
Change-Id: Ic2c9aadc47b76b73b97a1581c7c9379207e82ff7
(cherry picked from commit a30632c78dd2edc772517b594ea0d3389e7b1612)
Signed-off-by: Qais Yousef <qyousef@google.com>