ipvs: fix refcount usage for conns in ops mode

Connections in One-packet scheduling mode (-o, --ops) are
removed with refcnt=0 because they are not hashed in conn table.
To avoid refcount_dec reporting this as error, change them to be
removed with refcount_dec_if_one as all other connections.

refcount_t hit zero at ip_vs_conn_put+0x31/0x40 [ip_vs]
in sh[15519], uid/euid: 497/497
WARNING: CPU: 0 PID: 15519 at ../kernel/panic.c:657
refcount_error_report+0x94/0x9e
Modules linked in: ip_vs_rr cirrus ttm sb_edac
edac_core drm_kms_helper crct10dif_pclmul crc32_pclmul
ghash_clmulni_intel pcbc mousedev drm aesni_intel aes_x86_64
crypto_simd glue_helper cryptd psmouse evdev input_leds led_class
intel_agp fb_sys_fops syscopyarea sysfillrect intel_rapl_perf mac_hid
intel_gtt serio_raw sysimgblt agpgart i2c_piix4 i2c_core ata_generic
pata_acpi floppy cfg80211 rfkill button loop macvlan ip_vs
nf_conntrack libcrc32c crc32c_generic ip_tables x_tables ipv6
crc_ccitt autofs4 ext4 crc16 mbcache jbd2 fscrypto ata_piix libata
atkbd libps2 scsi_mod crc32c_intel i8042 rtc_cmos serio af_packet
dm_mod dax fuse xen_netfront xen_blkfront
CPU: 0 PID: 15519 Comm: sh Tainted: G        W
4.15.17 #1-NixOS
Hardware name: Xen HVM domU, BIOS 4.2.amazon 08/24/2006
RIP: 0010:refcount_error_report+0x94/0x9e
RSP: 0000:ffffa344dde039c8 EFLAGS: 00010296
RAX: 0000000000000057 RBX: ffffffff92f20e06 RCX: 0000000000000006
RDX: 0000000000000007 RSI: 0000000000000086 RDI: ffffa344dde165c0
RBP: ffffa344dde03b08 R08: 0000000000000218 R09: 0000000000000004
R10: ffffffff93006a80 R11: 0000000000000001 R12: ffffa344d68cd100
R13: 00000000000001f1 R14: ffffffff92f12fb0 R15: 0000000000000004
FS:  00007fc9d2040fc0(0000) GS:ffffa344dde00000(0000)
knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 000000000262a000 CR3: 0000000016a0c004 CR4: 00000000001606f0
Call Trace:
 <IRQ>
 ex_handler_refcount+0x4e/0x80
 fixup_exception+0x33/0x40
 do_trap+0x83/0x140
 do_error_trap+0x83/0xf0
 ? ip_vs_conn_drop_conntrack+0x120/0x1a5 [ip_vs]
 ? ip_finish_output2+0x29c/0x390
 ? ip_finish_output2+0x1a2/0x390
 invalid_op+0x1b/0x40
RIP: 0010:ip_vs_conn_put+0x31/0x40 [ip_vs]
RSP: 0000:ffffa344dde03bb8 EFLAGS: 00010246
RAX: 0000000000000001 RBX: ffffa344df31cf00 RCX: ffffa344d7450198
RDX: 0000000000000003 RSI: 00000000fffffe01 RDI: ffffa344d7450140
RBP: 0000000000000002 R08: 0000000000000476 R09: 0000000000000000
R10: ffffa344dde03b28 R11: ffffa344df200000 R12: ffffa344d7d09000
R13: ffffa344def3a980 R14: ffffffffc04f6e20 R15: 0000000000000008
 ip_vs_in.part.29.constprop.36+0x34f/0x640 [ip_vs]
 ? ip_vs_conn_out_get+0xe0/0xe0 [ip_vs]
 ip_vs_remote_request4+0x47/0xa0 [ip_vs]
 ? ip_vs_in.part.29.constprop.36+0x640/0x640 [ip_vs]
 nf_hook_slow+0x43/0xc0
 ip_local_deliver+0xac/0xc0
 ? ip_rcv_finish+0x400/0x400
 ip_rcv+0x26c/0x380
 __netif_receive_skb_core+0x3a0/0xb10
 ? inet_gro_receive+0x23c/0x2b0
 ? netif_receive_skb_internal+0x24/0xb0
 netif_receive_skb_internal+0x24/0xb0
 napi_gro_receive+0xb8/0xe0
 xennet_poll+0x676/0xb40 [xen_netfront]
 net_rx_action+0x139/0x3a0
 __do_softirq+0xde/0x2b4
 irq_exit+0xae/0xb0
 xen_evtchn_do_upcall+0x2c/0x40
 xen_hvm_callback_vector+0x7d/0x90
 </IRQ>
RIP: 0033:0x7fc9d11c91f9
RSP: 002b:00007ffebe8a2ea0 EFLAGS: 00000202 ORIG_RAX:
ffffffffffffff0c
RAX: 00000000ffffffff RBX: 0000000002609808 RCX: 0000000000000054
RDX: 0000000000000001 RSI: 0000000002605440 RDI: 00000000025f940e
RBP: 00000000025f940e R08: 000000000260213d R09: 1999999999999999
R10: 000000000262a808 R11: 00000000025f942d R12: 00000000025f940e
R13: 00007fc9d1301e20 R14: 00000000025f9408 R15: 00007fc9d1302720
Code: 48 8b 95 80 00 00 00 41 55 49 8d 8c 24 e0 05 00
00 45 8b 84 24 38 04 00 00 41 89 c1 48 89 de 48 c7 c7 a8 2f f2 92 e8
7c fa ff ff <0f> 0b 58 5b 5d 41 5c 41 5d c3 0f 1f 44 00 00 55 48 89 e5
41 56

Reported-by: Net Filter <netfilternetfilter@gmail.com>
Fixes: b54ab92b ("netfilter: refcounter conversions")
Signed-off-by: Julian Anastasov <ja@ssi.bg>
Acked-by: Simon Horman <horms@verge.net.au>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>