wait: avoid spurious calls to prepare_to_wait_event() in ___wait_event()

wait: avoid spurious calls to prepare_to_wait_event() in ___wait_event()

In vast majority of cases the condition determining whether the thread
can proceed is true after the first wake up.

However, even in that case the thread ends up calling into
prepare_to_wait_event() again, suffering a spurious irq + lock trip.

Then it calls into finish_wait() to unlink itself.

Note that in case of a pending signal the work done by
prepare_to_wait_event() gets ignored even without the change.

pre-check the condition after waking up instead.

Stats gathared during a kernel build:
bpftrace -e 'kprobe:prepare_to_wait_event,kprobe:finish_wait \
		 { @[probe] = count(); }'

@[kprobe:finish_wait]: 392483
@[kprobe:prepare_to_wait_event]: 778690

As in calls to prepare_to_wait_event() almost double calls to
finish_wait(). This evens out with the patch.

Signed-off-by: Mateusz Guzik <mjguzik@gmail.com>
Link: https://lore.kernel.org/r/20250303230409.452687-4-mjguzik@gmail.com
Signed-off-by: Christian Brauner <brauner@kernel.org>