ipvs: properly dereference pe in ip_vs_add_service

Use pe directly to resolve sparse warning:

  net/netfilter/ipvs/ip_vs_ctl.c:1471:27: warning: dereference of noderef expression

Fixes: 39b97223

 ("ipvs: handle connections started by real-servers")
Signed-off-by: Chen Hanxiao <chenhx.fnst@fujitsu.com>
Acked-by: Julian Anastasov <ja@ssi.bg>
Acked-by: Simon Horman <horms@kernel.org>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>