BUG/MINOR: dns: fix dangling dgram pointer on dns_dgram_init() failure path

BUG/MINOR: dns: fix dangling dgram pointer on dns_dgram_init() failure path

In dns_dgram_init(), the newly created dgram is assigned to the name server
before the ring is attached. In case of errors, e.g. due to too many watchers,
the dgram is released but not removed from ns->dgram. Let's only assign the
pointer on success to avoid this, as it's not needed before. The problem
was introduced in 2.4 with commit c943799c86 ("MEDIUM: resolvers/dns: split
dns.c into dns.c and resolvers.c"), and was possibly there before. The fix
may be backported to all stable versions.