ipvs: Add proper dependencies on IP_VS, and fix description header line.

Linus noted a build failure case:

net/netfilter/ipvs/ip_vs_xmit.c: In function 'ip_vs_tunnel_xmit':
net/netfilter/ipvs/ip_vs_xmit.c:616: error: implicit declaration of function 'ip_select_ident'

The proper include file (net/ip.h) is being included in ip_vs_xmit.c to get
that declaration.  So the only possible case where this can happen is if
CONFIG_INET is not enabled.

This seems to be purely a missing dependency in the ipvs/Kconfig file IP_VS
entry.

Also, while we're here, remove the out of date "EXPERIMENTAL" string in the
IP_VS config help header line.  IP_VS no longer depends upon CONFIG_EXPERIMENTAL

Signed-off-by: David S. Miller <davem@davemloft.net>