osd/scrub: add a variable to store osd_deep_scrub_stride

osd/scrub: add a variable to store osd_deep_scrub_stride

Use another variable to store the value of osd_deep_scrub_stride for later use
because osd_deep_scrub_stride can be modified on the fly, otherwise we may get
an incorrect digest value that contains only part of the object data
when osd_deep_scrub_stride is modified and it will make pg inconsistent.

Fixes: https://tracker.ceph.com/issues/58240
Signed-off-by: Zhansong Gao <zhsgao@hotmail.com>