IPVS: Minimise ip_vs_leave when CONFIG_SYSCTL is undefined

Much of ip_vs_leave() is unnecessary if CONFIG_SYSCTL is undefined.

I tried an approach of breaking the now #ifdef'ed portions out
into a separate function. However this appeared to grow the
compiled code on x86_64 by about 200 bytes in the case where
CONFIG_SYSCTL is defined. So I have gone with the simpler though
less elegant #ifdef'ed solution for now.

Signed-off-by: Simon Horman <horms@verge.net.au>