Merge branch 'net-initialize-fastreuse-on-inet_inherit_port'

Tim Froidcoeur says:

====================
net: initialize fastreuse on inet_inherit_port

In the case of TPROXY, bind_conflict optimizations for SO_REUSEADDR or
SO_REUSEPORT are broken, possibly resulting in O(n) instead of O(1) bind
behaviour or in the incorrect reuse of a bind.

the kernel keeps track for each bind_bucket if all sockets in the
bind_bucket support SO_REUSEADDR or SO_REUSEPORT in two fastreuse flags.
These flags allow skipping the costly bind_conflict check when possible
(meaning when all sockets have the proper SO_REUSE option).

For every socket added to a bind_bucket, these flags need to be updated.
As soon as a socket that does not support reuse is added, the flag is
set to false and will never go back to true, unless the bind_bucket is
deleted.

Note that there is no mechanism to re-evaluate these flags when a socket
is removed (this might make sense when removing a socket that would not
allow reuse; this leaves room for a future patch).

For this optimization to work, it is mandatory that these flags are
properly initialized and updated.

When a child socket is created from a listen socket in
__inet_inherit_port, the TPROXY case could create a new bind bucket
without properly initializing these flags, thus preventing the
optimization to work. Alternatively, a socket not allowing reuse could
be added to an existing bind bucket without updating the flags, causing
bind_conflict to never be called as it should.

Patch 1/2 refactors the fastreuse update code in inet_csk_get_port into a
small helper function, making the actual fix tiny and easier to understand.

Patch 2/2 calls this new helper when __inet_inherit_port decides to create
a new bind_bucket or use a different bind_bucket than the one of the listen
socket.

v4: - rebase on latest linux/net master branch
v3: - remove company disclaimer from automatic signature
v2: - remove unnecessary cast
====================

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