watch_queue: Use local kmap in post_one_notification()

watch_queue: Use local kmap in post_one_notification()

Replace the now deprecated kmap_atomic() with kmap_local_page().

Optimize for the non-highmem cases and avoid disabling preemption and
pagefaults, the caller's context is atomic anyway, but that is irrelevant
to kmap. The memcpy itself does not require any such semantics and the
mapping would hold valid across context switches anyway. Further, highmem
is planned to to be removed[1].

[1] https://lore.kernel.org/all/4ff89b72-03ff-4447-9d21-dd6a5fe1550f@app.fastmail.com/

Signed-off-by: Davidlohr Bueso <dave@stgolabs.net>
Link: https://patch.msgid.link/20251118210706.1816303-1-dave@stgolabs.net
Signed-off-by: Christian Brauner <brauner@kernel.org>