netfilter: nft_set_bitmap: free dummy elements when destroy the set

We forget to free dummy elements when deleting the set. So when I was
running nft-test.py, I saw many kmemleak warnings:
  kmemleak: 1344 new suspected memory leaks ...
  # cat /sys/kernel/debug/kmemleak
  unreferenced object 0xffff8800631345c8 (size 32):
  comm "nft", pid 9075, jiffies 4295743309 (age 1354.815s)
  hex dump (first 32 bytes):
    f8 63 13 63 00 88 ff ff 88 79 13 63 00 88 ff ff  .c.c.....y.c....
    04 0c 00 00 00 00 00 00 00 00 00 00 08 03 00 00  ................
  backtrace:
    [<ffffffff819059da>] kmemleak_alloc+0x4a/0xa0
    [<ffffffff81288174>] __kmalloc+0x164/0x310
    [<ffffffffa061269d>] nft_set_elem_init+0x3d/0x1b0 [nf_tables]
    [<ffffffffa06130da>] nft_add_set_elem+0x45a/0x8c0 [nf_tables]
    [<ffffffffa0613645>] nf_tables_newsetelem+0x105/0x1d0 [nf_tables]
    [<ffffffffa05fe6d4>] nfnetlink_rcv+0x414/0x770 [nfnetlink]
    [<ffffffff817f0ca6>] netlink_unicast+0x1f6/0x310
    [<ffffffff817f10c6>] netlink_sendmsg+0x306/0x3b0
  ...

Fixes: e920dde5 ("netfilter: nft_set_bitmap: keep a list of dummy elements")
Signed-off-by: Liping Zhang <zlpnobody@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>