The zone_refreshkeys() could run before the zone_shutdown(), but after
the last .erefs has been "detached" causing assertion failure when doing
dns_zone_attach(). Remove the use of .erefs (dns_zone_attach/detach)
and replace it with using the .irefs and additional checks whether the
zone is exiting in the callbacks.
(cherry picked from commit 80e66fbd2d)
Ondřej Surý
21d8d5e07d