ipvs: generalize app registration in netns

	Get rid of the ftp_app pointer and allow applications
to be registered without adding fields in the netns_ipvs structure.

v2: fix coding style as suggested by Pablo Neira Ayuso <pablo@netfilter.org>

Signed-off-by: Julian Anastasov <ja@ssi.bg>
Signed-off-by: Simon Horman <horms@verge.net.au>