netns: mib6 section fixlet

  LD      net/ipv6/ipv6.o
WARNING: net/ipv6/ipv6.o(.text+0xd8): Section mismatch in reference from the function inet6_net_init() to the function .init.text:ipv6_init_mibs()

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