ipvs: Fix uninit-value in do_ip_vs_set_ctl()

ipvs: Fix uninit-value in do_ip_vs_set_ctl()

do_ip_vs_set_ctl() is referencing uninitialized stack value when `len` is
zero. Fix it.

Reported-by: syzbot+23b5f9e7caf61d9a3898@syzkaller.appspotmail.com
Link: https://syzkaller.appspot.com/bug?id=46ebfb92a8a812621a001ef04d90dfa459520fe2
Suggested-by: Julian Anastasov <ja@ssi.bg>
Signed-off-by: Peilin Ye <yepeilin.cs@gmail.com>
Acked-by: Julian Anastasov <ja@ssi.bg>
Reviewed-by: Simon Horman <horms@verge.net.au>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>