netns: fix double free at netns creation

This patch fix a double free when a network namespace fails.
The previous code does a kfree of the net_generic structure when
one of the init subsystem initialization fails.
The 'setup_net' function does kfree(ng) and returns an error.
The caller, 'copy_net_ns', call net_free on error, and this one
calls kfree(net->gen), making this pointer freed twice.

This patch make the code symetric, the net_alloc does the net_generic
allocation and the net_free frees the net_generic.

Signed-off-by: Daniel Lezcano <daniel.lezcano@free.fr>
Signed-off-by: David S. Miller <davem@davemloft.net>