netdevsim: fib: Convert the current occupancy to an atomic variable

When route is added/deleted, the appropriate counter is increased/decreased
to maintain number of routes.

User can limit the number of routes and then according to the appropriate
counter, adding more routes than the limitation is forbidden.

Currently, there is one lock which protects hashtable, list and accounting.

Handling the counters will be performed from both atomic context and
non-atomic context, while the hashtable and the list will be used only from
non-atomic context and therefore will be protected by a separate lock.

Protect accounting by using an atomic variable, so lock is not needed.

v2:
* Use atomic64_sub() in nsim_nexthop_account()'s error path

Signed-off-by: Amit Cohen <amcohen@nvidia.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>