ipv6: addrlabel: fix possible memory leak in ip6addrlbl_net_init

kmemleak report a memory leak as follows:

unreferenced object 0xffff8880059c6a00 (size 64):
  comm "ip", pid 23696, jiffies 4296590183 (age 1755.384s)
  hex dump (first 32 bytes):
    20 01 00 10 00 00 00 00 00 00 00 00 00 00 00 00   ...............
    1c 00 00 00 00 00 00 00 00 00 00 00 07 00 00 00  ................
  backtrace:
    [<00000000aa4e7a87>] ip6addrlbl_add+0x90/0xbb0
    [<0000000070b8d7f1>] ip6addrlbl_net_init+0x109/0x170
    [<000000006a9ca9d4>] ops_init+0xa8/0x3c0
    [<000000002da57bf2>] setup_net+0x2de/0x7e0
    [<000000004e52d573>] copy_net_ns+0x27d/0x530
    [<00000000b07ae2b4>] create_new_namespaces+0x382/0xa30
    [<000000003b76d36f>] unshare_nsproxy_namespaces+0xa1/0x1d0
    [<0000000030653721>] ksys_unshare+0x3a4/0x780
    [<0000000007e82e40>] __x64_sys_unshare+0x2d/0x40
    [<0000000031a10c08>] do_syscall_64+0x33/0x40
    [<0000000099df30e7>] entry_SYSCALL_64_after_hwframe+0x44/0xa9

We should free all rules when we catch an error in ip6addrlbl_net_init().
otherwise a memory leak will occur.

Fixes: 2a8cc6c8

 ("[IPV6] ADDRCONF: Support RFC3484 configurable address selection policy table.")
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Wang Hai <wanghai38@huawei.com>
Link: https://lore.kernel.org/r/20201124071728.8385-1-wanghai38@huawei.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>