netfilter: nf_log_syslog: Unset bridge logger in pernet exit

Without this, a stale pointer remains in pernet loggers after module
unload causing a kernel oops during dereference. Easily reproduced by:

| # modprobe nf_log_syslog
| # rmmod nf_log_syslog
| # cat /proc/net/netfilter/nf_log

Fixes: 77ccee96

 ("netfilter: nf_log_bridge: merge with nf_log_syslog")
Signed-off-by: Phil Sutter <phil@nwl.cc>
Acked-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>