ipvs: fix sparse warnings for ip_vs_conn listing

kbuild test robot reports for sparse warnings
in commit 088339a5 ("ipvs: convert connection locking"):

net/netfilter/ipvs/ip_vs_conn.c:962:13: warning: context imbalance
	in 'ip_vs_conn_array' - wrong count at exit
include/linux/rcupdate.h:326:30: warning: context imbalance in
	'ip_vs_conn_seq_next' - unexpected unlock
include/linux/rcupdate.h:326:30: warning: context imbalance in
	'ip_vs_conn_seq_stop' - unexpected unlock

Fix it by running ip_vs_conn_array under RCU lock
to avoid conditional locking and by adding proper RCU
annotations.

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