scope security check for the UDPv6 socket credential lookup service,
allowing security policies to bound access to credential information.
While not an immediate issue for Jail, which doesn't allow use of UDPv6,
this may be relevant to other security policies that may wish to control
ident lookups.
While here, eliminate a very unlikely panic case, in which a socket in
the process of being freed is inspected by the sysctl.
Approved by: re (kensmith)
Reviewed by: bz
9e7a99e592