mountd: parse ip address earlier

I don't see the point of waiting to the last minute to parse the ip
address.  If the client name isn't a legal ip address then this will
fail fairly quickly, so there's not much of a performance penalty.

Also, note the previous code incorrectly assumed client_resolve would
always return non-NULL.

Also factor out some common code.

Signed-off-by: J. Bruce Fields <bfields@redhat.com>
Signed-off-by: Steve Dickson <steved@redhat.com>