atm: fix a memory leak of vcc->user_back

atm: fix a memory leak of vcc->user_back

In lec_arp_clear_vccs() only entry->vcc is freed, but vcc
could be installed on entry->recv_vcc too in lec_vcc_added().

This fixes the following memory leak:

unreferenced object 0xffff8880d9266b90 (size 16):
  comm "atm2", pid 425, jiffies 4294907980 (age 23.488s)
  hex dump (first 16 bytes):
    00 00 00 00 00 00 00 00 00 00 00 00 6b 6b 6b a5  ............kkk.
  backtrace:
    [<(____ptrval____)>] kmem_cache_alloc_trace+0x10e/0x151
    [<(____ptrval____)>] lane_ioctl+0x4b3/0x569
    [<(____ptrval____)>] do_vcc_ioctl+0x1ea/0x236
    [<(____ptrval____)>] svc_ioctl+0x17d/0x198
    [<(____ptrval____)>] sock_do_ioctl+0x47/0x12f
    [<(____ptrval____)>] sock_ioctl+0x2f9/0x322
    [<(____ptrval____)>] vfs_ioctl+0x1e/0x2b
    [<(____ptrval____)>] ksys_ioctl+0x61/0x80
    [<(____ptrval____)>] __x64_sys_ioctl+0x16/0x19
    [<(____ptrval____)>] do_syscall_64+0x57/0x65
    [<(____ptrval____)>] entry_SYSCALL_64_after_hwframe+0x49/0xb3

Cc: Gengming Liu <l.dmxcsnsbh@gmail.com>
Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>