librbd/cache/pwl: arm periodic stats timer only after init succeeds

If an init step fails after pwl_init() armed the stats timer,
InitRequest deletes the image cache without shut_down(), so the
self-rearming timer callback keeps firing on the freed object.

Arm the timer after write_image_cache_state() completes instead: on
failure paths it is never armed, and any later teardown goes through
shut_down(), which cancels it.

Fixes: https://tracker.ceph.com/issues/79548

Signed-off-by: Sun Yuechi <sunyuechi@iscas.ac.cn>