virarptable: Fix check for message length

virarptable: Fix check for message length

The previous check was all wrong since it calculated the how long would
the netlink message be if the netlink header was the payload and then
subtracted that from the whole message length, a variable that was not
used later in the code.  This check can fail if there are no additional
payloads, struct rtattr in particular, which we are parsing later,
however the RTA_OK macro would've caught that anyway.

Signed-off-by: Martin Kletzander <mkletzan@redhat.com>
Reviewed-by: Laine Stump <laine@redhat.com>