proc: clean the ip_misc_proc_init and ip_proc_init_net error paths

After all this stuff is moved outside, this function can look better.

Besides, I tuned the error path in ip_proc_init_net to make it have
only 2 exit points, not 3.

Signed-off-by: Pavel Emelyanov <xemul@openvz.org>
Signed-off-by: David S. Miller <davem@davemloft.net>