tipc: add stricter control of reserved service types

tipc: add stricter control of reserved service types

TIPC reserves 64 service types for current and future internal use.
Therefore, the bind() function is meant to block regular user sockets
from being bound to these values, while it should let through such
bindings from internal users.

However, since we at the design moment saw no way to distinguish
between regular and internal users the filter function ended up
with allowing all bindings of the reserved types which were really
in use ([0,1]), and block all the rest ([2,63]).

This is risky, since a regular user may bind to the service type
representing the topology server (TIPC_TOP_SRV == 1) or the one used
for indicating neighboring node status (TIPC_CFG_SRV == 0), and wreak
havoc for users of those services, i.e., most users.

The reality is however that TIPC_CFG_SRV never is bound through the
bind() function, since it doesn't represent a regular socket, and
TIPC_TOP_SRV can also be made to bypass the checks in tipc_bind()
by introducing a different entry function, tipc_sk_bind().

It should be noted that although this is a change of the API semantics,
there is no risk we will break any currently working applications by
doing this. Any application trying to bind to the values in question
would be badly broken from the outset, so there is no chance we would
find any such applications in real-world production systems.

v2: Added warning printout when a user is blocked from binding,
    as suggested by Jakub Kicinski

Acked-by: Yung Xue <ying.xue@windriver.com>
Signed-off-by: Jon Maloy <jmaloy@redhat.com>
Link: https://lore.kernel.org/r/20201030012938.489557-1-jmaloy@redhat.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>