[NETNS]: Fix export symbols.

Add the appropriate EXPORT_SYMBOLS for proc_net_create,
proc_net_fops_create and proc_net_remove to fix errors when
compiling allmodconfig

Signed-off-by: Mark Nelson <markn@au1.ibm.com>
Acked-by: Benjamin Thery <benjamin.thery@bull.net>
Signed-off-by: David S. Miller <davem@davemloft.net>