netfilter: nft_log: perform module load from nf_tables

modprobe calls from the nf_logger_find_get() API causes deadlock in very
special cases because they occur with the nf_tables transaction mutex held.

In the specific case of nf_log, deadlock is via:

 A nf_tables -> transaction mutex -> nft_log -> modprobe -> nf_log_syslog \
	    -> pernet_ops rwsem -> wait for C
 B netlink event -> rtnl_mutex -> nf_tables transaction mutex -> wait for A
 C close() -> ip6mr_sk_done -> rtnl_mutex -> wait for B

Earlier patch added NFLOG/xt_LOG module softdeps to avoid the need to load
the backend module during a transaction.

For nft_log we would have to add a softdep for both nfnetlink_log or
nf_log_syslog, since we do not know in advance which of the two backends
are going to be configured.

This defers the modprobe op until after the transaction mutex is released.

Tested-by: Phil Sutter <phil@nwl.cc>
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>