[CIFS] clean up upcall handling for dns_resolver keys
We're given the datalen in the downcall, so there's no need to do any
calls to strlen(). Just keep track of the datalen in the key. Finally,
add a sanity check of the data in the downcall to make sure that it
looks like a real IP address.
Signed-off-by: Jeff Layton <jlayton@redhat.com> Acked-by: David Howells <dhowells@redhat.com> Signed-off-by: Steve French <sfrench@us.ibm.com>