inet: frags: reorganize struct netns_frags

Put the read-mostly fields in a separate cache line
at the beginning of struct netns_frags, to reduce
false sharing noticed in inet_frag_kill()

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
(cherry picked from commit c2615cf5)
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>