netfilter: nf_log: fix sparse warning in nf_logger_find_get()

net/netfilter/nf_log.c:157:16: warning: incorrect type in assignment (different address spaces)
net/netfilter/nf_log.c:157:16:    expected struct nf_logger *logger
net/netfilter/nf_log.c:157:16:    got struct nf_logger [noderef] <asn:4>*<noident>

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>