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+0x178/0...