netfilter: nf_conncount: use spin_lock_bh instead of spin_lock

conn_free() holds lock with spin_lock() and it is called by both
nf_conncount_lookup() and nf_conncount_gc_list(). nf_conncount_lookup()
is called from bottom-half context and nf_conncount_gc_list() from
process context. So that spin_lock() call is not safe. Hence
conn_free() should use spin_lock_bh() instead of spin_lock().

test commands:
   %nft add table ip filter
   %nft add chain ip filter input { type filter hook input priority 0\; }
   %nft add rule filter input meter test { ip saddr ct count over 2 } \
	   counter

splat looks like:
[  461.996507] ================================
[  461.998999] WARNING: inconsistent lock state
[  461.998999] 4.19.0-rc6+ #22 Not tainted
[  461.998999] --------------------------------
[  461.998999] inconsistent {IN-SOFTIRQ-W} -> {SOFTIRQ-ON-W} usage.
[  461.998999] kworker/0:2/134 [HC0[0]:SC0[0]:HE1:SE1] takes:
[  461.998999] 00000000a71a559a (&(&list->list_lock)->rlock){+.?.}, at: conn_free+0x69/0x2b0 [nf_conncount]
[  461.998999] {IN-SOFTIRQ-W} state was registered at:
[  461.998999]   _raw_spin_lock+0x30/0x70
[  461.998999]   nf_conncount_add+0x28a/0x520 [nf_conncount]
[  461.998999]   nft_connlimit_eval+0x401/0x580 [nft_connlimit]
[  461.998999]   nft_dynset_eval+0x32b/0x590 [nf_tables]
[  461.998999]   nft_do_chain+0x497/0x1430 [nf_tables]
[  461.998999]   nft_do_chain_ipv4+0x255/0x330 [nf_tables]
[  461.998999]   nf_hook_slow+0xb1/0x160
[ ... ]
[  461.998999] other info that might help us debug this:
[  461.998999]  Possible unsafe locking scenario:
[  461.998999]
[  461.998999]        CPU0
[  461.998999]        ----
[  461.998999]   lock(&(&list->list_lock)->rlock);
[  461.998999]   <Interrupt>
[  461.998999]     lock(&(&list->list_lock)->rlock);
[  461.998999]
[  461.998999]  *** DEADLOCK ***
[  461.998999]
[ ... ]

Fixes: 5c789e13 ("netfilter: nf_conncount: Add list lock and gc worker, and RCU for init tree search")
Signed-off-by: Taehee Yoo <ap420073@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>