net: Convert uevent_net_ops

net: Convert uevent_net_ops

uevent_net_init() and uevent_net_exit() create and
destroy netlink socket, and these actions serialized
in netlink code.

Parallel execution with other pernet_operations
makes the socket disappear earlier from uevent_sock_list
on ->exit. As userspace can't be interested in broadcast
messages of dying net, and, as I see, no one in kernel
listen them, we may safely make uevent_net_ops async.

Signed-off-by: Kirill Tkhai <[email protected]>
Acked-by: Andrei Vagin <[email protected]>
Signed-off-by: David S. Miller <[email protected]>