net: shy netns_ok check

Failure to pass netns_ok check is SILENT, except some MIB counter is
incremented somewhere.

And adding "netns_ok = 1" (after long head-scratching session) is
usually the last step in making some protocol netns-ready...

Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>