[IPVS]: Make sure ip_vs_ftp ports are valid

I'm not entirely sure what happens in the case of a valid port,
at best it'll be silently ignored. This patch ignores them a little
more verbosely.

Signed-Off-By: Simon Horman <horms@verge.net.au>
Signed-off-by: David S. Miller <davem@davemloft.net>