ipvs: fix possible memory leak in ip_vs_control_net_init

kmemleak report a memory leak as follows:

BUG: memory leak
unreferenced object 0xffff8880759ea000 (size 256):
backtrace:
[<00000000c0bf2deb>] kmem_cache_zalloc include/linux/slab.h:656 [inline]
[<00000000c0bf2deb>] __proc_create+0x23d/0x7d0 fs/proc/generic.c:421
[<000000009d718d02>] proc_create_reg+0x8e/0x140 fs/proc/generic.c:535
[<0000000097bbfc4f>] proc_create_net_data+0x8c/0x1b0 fs/proc/proc_net.c:126
[<00000000652480fc>] ip_vs_control_net_init+0x308/0x13a0 net/netfilter/ipvs/ip_vs_ctl.c:4169
[<000000004c927ebe>] __ip_vs_init+0x211/0x400 net/netfilter/ipvs/ip_vs_core.c:2429
[<00000000aa6b72d9>] ops_init+0xa8/0x3c0 net/core/net_namespace.c:151
[<00000000153fd114>] setup_net+0x2de/0x7e0 net/core/net_namespace.c:341
[<00000000be4e4f07>] copy_net_ns+0x27d/0x530 net/core/net_namespace.c:482
[<00000000f1c23ec9>] create_new_namespaces+0x382/0xa30 kernel/nsproxy.c:110
[<00000000098a5757>] copy_namespaces+0x2e6/0x3b0 kernel/nsproxy.c:179
[<0000000026ce39e9>] copy_process+0x220a/0x5f00 kernel/fork.c:2072
[<00000000b71f4efe>] _do_fork+0xc7/0xda0 kernel/fork.c:2428
[<000000002974ee96>] __do_sys_clone3+0x18a/0x280 kernel/fork.c:2703
[<0000000062ac0a4d>] do_syscall_64+0x33/0x40 arch/x86/entry/common.c:46
[<0000000093f1ce2c>] entry_SYSCALL_64_after_hwframe+0x44/0xa9

In the error path of ip_vs_control_net_init(), remove_proc_entry() needs
to be called to remove the added proc entry, otherwise a memory leak
will occur.

Also, add some '#ifdef CONFIG_PROC_FS' because proc_create_net* return NULL
when PROC is not used.

Fixes: b17fc996 ("IPVS: netns, ip_vs_stats and its procfs")
Fixes: 61b1ab45 ("IPVS: netns, add basic init per netns.")
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Wang Hai <wanghai38@huawei.com>
Acked-by: Julian Anastasov <ja@ssi.bg>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>