netdevsim: fib: Introduce a lock to guard nexthop hashtable

Currently netdevsim relies on RTNL to maintain exclusivity in accessing the
nexthop hash table. However, bucket notification may be called without RTNL
having been held. Instead, introduce a custom lock to guard the table.

Signed-off-by: Petr Machata <petrm@nvidia.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>