Merge branch 'Replacing-net_mutex-with-rw_semaphore'

Kirill Tkhai says:

====================
Replacing net_mutex with rw_semaphore

this is the third version of the patchset introducing net_sem
instead of net_mutex. The patchset adds net_sem in addition
to net_mutex and allows pernet_operations to be "async". This
flag means, the pernet_operations methods are safe to be
executed with any other pernet_operations (un)initializing
another net.

If there are only async pernet_operations in the system,
net_mutex is not used either for setup_net() or for cleanup_net().

The pernet_operations converted in this patchset allow
to create minimal .config to have network working, and
the changes improve the performance like you may see
below:

    %for i in {1..10000}; do unshare -n bash -c exit; done

    *before*
    real 1m40,377s
    user 0m9,672s
    sys 0m19,928s

    *after*
    real 0m17,007s
    user 0m5,311s
    sys 0m11,779

    (5.8 times faster)

In the future, when all pernet_operations become async,
we'll just remove this "async" field tree-wide.

All the new logic is concentrated in patches [1-5/32].
The rest of patches converts specific operations:
review, rationale of they can be converted, and setting
of async flag.

Kirill

v3: Improved patches descriptions. Added comment into [5/32].
Added [32/32] converting netlink_tap_net_ops (new pernet operations
introduced in 2018).

v2: Single patch -> patchset with rationale of every conversion
====================

Signed-off-by: David S. Miller <davem@davemloft.net>