librbd/cache/pwl: Fix dead lock issue when pwl initialization failed

librbd/cache/pwl: Fix dead lock issue when pwl initialization failed

when pwl initialization failed, 'AbstractWriteLog' will release itself
in callback, it hold guard lock and want to get lock to delete data,
which causes dead lock. This PR works by release image_cache outside
the callback function.

Signed-off-by: Yin Congmin <congmin.yin@intel.com>
(cherry picked from commit 937af36e204791554708632245b4bca1d52f49a6)