Due to off-by-one error in zonemgr_keymgmt_delete, unused key file IO
lock objects were never freed and they were kept until the server
shutdown. Adjust the returned value by -1 to accomodate the fact that
the atomic_fetch_*() functions return the value before the operation and
not current value after the operation.
(cherry picked from commit fb1acd6736)
98fca774b6