mib: add netns/mib.h file

The only structure declared within is the netns_mib, which will
carry all our mibs within. I didn't put the mibs in the existing
netns_xxx structures to make it possible to mark this one as
properly aligned and get in a separate "read-mostly" cache-line.

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