Fix "rndc flushname" for longer name server names

Fix "rndc flushname" for longer name server names

dns_adb_flushname() calls dns_name_hash() to determine the ADB bucket
number to search for the given name.  Meanwhile, all other functions in
lib/dns/adb.c call dns_name_fullhash() for determining the bucket number
instead.  This discrepancy causes dns_adb_flushname() to have virtually
no chances of actually removing the given name from the ADB if the
name is longer than 16 bytes (since dns_name_hash() only hashes the
first 16 bytes of the name provided to it) - more specifically, the
probability of success for names longer than 16 bytes is inversely
proportional to the number of ADB buckets in use, i.e. 1:1021 at best.

Fix by using dns_name_fullhash() instead of dns_name_hash() in
dns_adb_flushname(), so that the logic for determining the bucket number
that a given name belongs to is consistent throughout lib/dns/adb.c.