ipvs: Expose ip_vs_ftp module parameters via sysfs.

This is to expose "ports" parameter via sysfs so it can be read
at any time in order to determine what port or ports were passed
to the module at the point when it was loaded.

Signed-off-by: Krzysztof Wilczynski <krzysztof.wilczynski@linux.com>
Signed-off-by: Simon Horman <horms@verge.net.au>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>