Since net could be obtained from RCU lists,
and there is a race with net destruction,
the patch converts net::count to refcount_t.

This provides sanity checks for the cases of
incrementing counter of already dead net,
when maybe_get_net() has to used instead
of get_net().

Drivers: allyesconfig and allmodconfig are OK.
Suggested-by: NEric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: NKirill Tkhai <ktkhai@virtuozzo.com>
Reviewed-by: NEric Dumazet <edumazet@google.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>