[9.20] fix: dev: Add a missing rcu_read_unlock() call on exit path
requested to merge backport-aram/add-missing-rcu_read_unlock-in-dns_dispatch_add-9.20 into bind-9.20
An exit path in the dns_dispatch_add() function fails to get out of the RCU critical section when returning early. Add the missing rcu_read_unlock() call.
Backport of MR !9561 (merged)