[9.20] fix: dev: Add a missing rcu_read_unlock() call on exit path

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

Merge branch 'backport-aram/add-missing-rcu_read_unlock-in-dns_dispatch_add-9.20' into 'bind-9.20'

See merge request isc-projects/bind9!9564