IPVS: use pr_err and friends instead of IP_VS_ERR and friends

Since pr_err and friends are used instead of printk there is no point
in keeping IP_VS_ERR and friends.  Furthermore make use of '__func__'
instead of hard coded function names.

Signed-off-by: Hannes Eder <heder@google.com>
Acked-by: Simon Horman <horms@verge.net.au>
Signed-off-by: David S. Miller <davem@davemloft.net>