ipvs: properly dereference dest_dst in ip_vs_forget_dev

Use rcu_dereference_protected to resolve
sparse warning, found by kbuild test robot:

net/netfilter/ipvs/ip_vs_ctl.c:1464:35: warning: dereference of
noderef expression

Problem from commit 026ace06
("ipvs: optimize dst usage for real server")

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