The dns_zonemgr_releasezone() function makes a decision to destroy
'zmgr' (based on its references count, after decreasing it) inside
a lock, and then destroys the object outside of the lock.
This causes a race with dns_zonemgr_detach(), which could destroy
the object in the meantime.
Change dns_zonemgr_releasezone() to detach from 'zmgr' and destroy
the object (if needed) using dns_zonemgr_detach(), outside of the
lock.
(cherry picked from commit c1fc212253)
272afcd999