netfilter: Fix memory leak in nf_register_net_hook

In the rare case that when it is a attempted to use a per network device
netfilter hook and the network device does not exist the newly allocated
structure can leak.

Be a good citizen and free the newly allocated structure in the error
handling code.

Fixes: 085db2c0

 ("netfilter: Per network namespace netfilter hooks.")
Reported-by:  <kbuild@01.org>
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>