ipvs: Restrict connection table size via Kconfig

Instead of checking the value in include/net/ip_vs.h, we can just
restrict the range in our Kconfig file. This will prevent values outside
of the range early.

Signed-off-by: Sven Wegener <sven.wegener@stealer.net>
Reviewed-by: Julius Volz <juliusv@google.com>
Signed-off-by: Simon Horman <horms@verge.net.au>