librbd/cache/pwl: adjust compare-and-write compare substringing

librbd/cache/pwl: adjust compare-and-write compare substringing

cmp_bl can no longer be shorter than read_bl (i.e. image extent).
This is caught very early at the API level, see commit af96e6dae3f4
("librbd: make C++ cmp&write semantics equal to C API").

However, cmp_bl can still be longer than read_bl and in that case
it should be capped by the image extent length.

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>