memory: bail from page scrubbing when CPU is no longer online

memory: bail from page scrubbing when CPU is no longer online

Scrubbing can significantly delay the offlining (parking) of a CPU (e.g.
because of booting into in smt=0 mode), to a degree that the "CPU <n>
still not dead..." messages logged on x86 in 1s intervals can be seen
multiple times. There are no softirqs involved in this process, so
extend the existing preemption check in the scrubbing logic to also exit
when the CPU is no longer observed online.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>