netfilter: nft_compat: fix crash when related match/target module is removed

We "cache" the loaded match/target modules and reuse them, but when the
modules are removed, we still point to them. Then we may end up with
invalid memory references when using iptables-compat to add rules later.

Input the following commands will reproduce the kernel crash:
  # iptables-compat -A INPUT -j LOG
  # iptables-compat -D INPUT -j LOG
  # rmmod xt_LOG
  # iptables-compat -A INPUT -j LOG
  BUG: unable to handle kernel paging request at ffffffffa05a9010
  IP: [<ffffffff813f783e>] strcmp+0xe/0x30
  Call Trace:
  [<ffffffffa05acc43>] nft_target_select_ops+0x83/0x1f0 [nft_compat]
  [<ffffffffa058a177>] nf_tables_expr_parse+0x147/0x1f0 [nf_tables]
  [<ffffffffa058e541>] nf_tables_newrule+0x301/0x810 [nf_tables]
  [<ffffffff8141ca00>] ? nla_parse+0x20/0x100
  [<ffffffffa057fa8f>] nfnetlink_rcv+0x33f/0x53d [nfnetlink]
  [<ffffffffa057f94b>] ? nfnetlink_rcv+0x1fb/0x53d [nfnetlink]
  [<ffffffff817116b8>] netlink_unicast+0x178/0x220
  [<ffffffff81711a5b>] netlink_sendmsg+0x2fb/0x3a0
  [<ffffffff816b7fc8>] sock_sendmsg+0x38/0x50
  [<ffffffff816b8a7e>] ___sys_sendmsg+0x28e/0x2a0
  [<ffffffff816bcb7e>] ? release_sock+0x1e/0xb0
  [<ffffffff81804ac5>] ? _raw_spin_unlock_bh+0x35/0x40
  [<ffffffff816bcbe2>] ? release_sock+0x82/0xb0
  [<ffffffff816b93d4>] __sys_sendmsg+0x54/0x90
  [<ffffffff816b9422>] SyS_sendmsg+0x12/0x20
  [<ffffffff81805172>] entry_SYSCALL_64_fastpath+0x1a/0xa9

So when nobody use the related match/target module, there's no need to
"cache" it. And nft_[match|target]_release are useless anymore, remove
them.

Signed-off-by: Liping Zhang <liping.zhang@spreadtrum.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>