netdevsim: Add simple FIB resource controller via devlink

Add devlink support to netdevsim and use it to implement a simple,
profile based resource controller. Only one controller is needed
per namespace, so the first netdevsim netdevice in a namespace
registers with devlink. If that device is deleted, the resource
settings are deleted.

The resource controller allows a user to limit the number of IPv4 and
IPv6 FIB entries and FIB rules. The resource paths are:
    /IPv4
    /IPv4/fib
    /IPv4/fib-rules
    /IPv6
    /IPv6/fib
    /IPv6/fib-rules

The IPv4 and IPv6 top level resources are unlimited in size and can not
be changed. From there, the number of FIB entries and FIB rule entries
are unlimited by default. A user can specify a limit for the fib and
fib-rules resources:

    $ devlink resource set netdevsim/netdevsim0 path /IPv4/fib size 96
    $ devlink resource set netdevsim/netdevsim0 path /IPv4/fib-rules size 16
    $ devlink resource set netdevsim/netdevsim0 path /IPv6/fib size 64
    $ devlink resource set netdevsim/netdevsim0 path /IPv6/fib-rules size 16
    $ devlink dev reload netdevsim/netdevsim0

such that the number of rules or routes is limited (96 ipv4 routes in the
example above):
    $ for n in $(seq 1 32); do ip ro add 10.99.$n.0/24 dev eth1; done
    Error: netdevsim: Exceeded number of supported fib entries.

    $ devlink resource show netdevsim/netdevsim0
    netdevsim/netdevsim0:
      name IPv4 size unlimited unit entry size_min 0 size_max unlimited size_gran 1 dpipe_tables non
        resources:
          name fib size 96 occ 96 unit entry size_min 0 size_max unlimited size_gran 1 dpipe_tables
    ...

With this template in place for resource management, it is fairly trivial
to extend and shows one way to implement a simple counter based resource
controller typical of network profiles.

Currently, devlink only supports initial namespace. Code is in place to
adapt netdevsim to a per namespace controller once the network namespace
issues are resolved.

Signed-off-by: David Ahern <dsa@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>