ipvs: use standardized format in sprintf

Use the same format string as net/ipv4/netfilter/nf_nat_ftp.c
to encode an ipv4 address and port.

Both uses should be a single common function.

Signed-off-by: Joe Perches <joe@perches.com>
Acked-by: Simon Horman <horms@verge.net.au>
Signed-off-by: Patrick McHardy <kaber@trash.net>