net: Convert atomic_t net::count to refcount_t

net: Convert atomic_t net::count to refcount_t

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: Eric Dumazet <[email protected]>
Signed-off-by: Kirill Tkhai <[email protected]>
Reviewed-by: Eric Dumazet <[email protected]>
Signed-off-by: David S. Miller <[email protected]>