ipvs: Wrap sysctl_cache_bypass and remove ifdefs in ip_vs_leave

With sysctl_cache_bypass now a compile time constant the compiler can
figue out that it can elimiate all of the code that depends on
sysctl_cache_bypass being true.

Also remove the duplicate computation of net previously necessitated
by #ifdef CONFIG_SYSCTL

Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
Acked-by: Julian Anastasov <ja@ssi.bg>
Signed-off-by: Simon Horman <horms@verge.net.au>