netfilter: log: protect nf_log_register against double registering

Currently, despite the comment right before the function,
nf_log_register allows registering two loggers on with the same type and
end up overwriting the previous register.

Not a real issue today as current tree doesn't have two loggers for the
same type but it's better to get this protected.

Also make sure that all of its callers do error checking.

Signed-off-by: Marcelo Ricardo Leitner <mleitner@redhat.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>