This significantly reduces overhead of dirty page
rate calculation in sampling mode.
Tested using 32GiB VM on E5-2690 CPU.

With CRC32:
total_pages=8388608 sampled_pages=16384 millis=71

With xxHash:
total_pages=8388608 sampled_pages=16384 millis=14

Signed-off-by: Andrei Gudkov <gudkov.andrei@huawei.com>
Message-Id: <cd115a89fc81d5f2eeb4ea7d57a98b84f794f340.1682598010.git.gudkov.andrei@huawei.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>