ipvs: Fix uninit-value in do_ip_vs_set_ctl()

[ Upstream commit c5a8a8498eed1c164afc94f50a939c1a10abf8ad ]

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>
Signed-off-by: Sasha Levin <sashal@kernel.org>