[NETNS]: Fix compiler error in net_namespace.c

Because net_free is called by copy_net_ns before its declaration, the
compiler gives an error. This patch puts net_free before copy_net_ns
to fix this.

The compiler error:
net/core/net_namespace.c: In function 'copy_net_ns':
net/core/net_namespace.c:97: error: implicit declaration of function 'net_free'
net/core/net_namespace.c: At top level:
net/core/net_namespace.c:104: warning: conflicting types for 'net_free'
net/core/net_namespace.c:104: error: static declaration of 'net_free' follows non-static declaration
net/core/net_namespace.c:97: error: previous implicit declaration of 'net_free' was here

The error was introduced by the '[NET]: Hide the dead code in the
net_namespace.c' patch (6a1a3b9f

).

Signed-off-by: Johann Felix Soden <johfel@users.sourceforge.net>
Signed-off-by: David S. Miller <davem@davemloft.net>