writecache: use wipe_lv to warn about specific signatures

writecache: use wipe_lv to warn about specific signatures

When initializing an LV to hold the writecache, use wipe_lv()
which looks for specific signatures on the LV.

Wiping signatures is not necessary, but printing a warning
that names a specific signature (in addition to the existing
generic warning/confirmation) may help if a user accidentally
specifies the wrong LV which contains something important.