datapath: Introduce net_rwsem and remove rtnl_lock()

datapath: Introduce net_rwsem and remove rtnl_lock()

This patch backports the following two upstream commits and
add a new symbol HAVE_NET_RWSEM in acinclude.m4 to determine
whether to use new introduced rw_semaphore, net_rwsem.

Upstream commit:
    commit f0b07bb151b098d291fd1fd71ef7a2df56fb124a
    Author: Kirill Tkhai <ktkhai@virtuozzo.com>
    Date:   Thu Mar 29 19:20:32 2018 +0300

    net: Introduce net_rwsem to protect net_namespace_list

    rtnl_lock() is used everywhere, and contention is very high.
    When someone wants to iterate over alive net namespaces,
    he/she has no a possibility to do that without exclusive lock.
    But the exclusive rtnl_lock() in such places is overkill,
    and it just increases the contention. Yes, there is already
    for_each_net_rcu() in kernel, but it requires rcu_read_lock(),
    and this can't be sleepable. Also, sometimes it may be need
    really prevent net_namespace_list growth, so for_each_net_rcu()
    is not fit there.

    This patch introduces new rw_semaphore, which will be used
    instead of rtnl_mutex to protect net_namespace_list. It is
    sleepable and allows not-exclusive iterations over net
    namespaces list. It allows to stop using rtnl_lock()
    in several places (what is made in next patches) and makes
    less the time, we keep rtnl_mutex. Here we just add new lock,
    while the explanation of we can remove rtnl_lock() there are
    in next patches.

    Fine grained locks generally are better, then one big lock,
    so let's do that with net_namespace_list, while the situation
    allows that.

    Signed-off-by: Kirill Tkhai <ktkhai@virtuozzo.com>
    Signed-off-by: David S. Miller <davem@davemloft.net>
Upstream commit:
    commit ec9c780925c57588637e1dbd8650d294107311c0
    Author: Kirill Tkhai <ktkhai@virtuozzo.com>
    Date:   Thu Mar 29 19:21:09 2018 +0300

    ovs: Remove rtnl_lock() from ovs_exit_net()

    Here we iterate for_each_net() and removes
    vport from alive net to the exiting net.

    ovs_net::dps are protected by ovs_mutex(),
    and the others, who change it (ovs_dp_cmd_new(),
    __dp_destroy()) also take it.
    The same with datapath::ports list.

    So, we remove rtnl_lock() here.

    Signed-off-by: Kirill Tkhai <ktkhai@virtuozzo.com>
    Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Yi-Hung Wei <yihung.wei@gmail.com>
Reviewed-by: Greg Rose <gvrose8192@gmail.com>
Signed-off-by: Justin Pettit <jpettit@ovn.org>