nexthop: Restart nexthop dump based on last dumped nexthop identifier

Currently, a multi-part nexthop dump is restarted based on the number of
nexthops that have been dumped so far. This can result in a lot of
nexthops not being dumped when nexthops are simultaneously deleted:

 # ip nexthop | wc -l
 65536
 # ip nexthop flush
 Dump was interrupted and may be inconsistent.
 Flushed 36040 nexthops
 # ip nexthop | wc -l
 29496

Instead, restart the dump based on the nexthop identifier (fixed number)
of the last successfully dumped nexthop:

 # ip nexthop | wc -l
 65536
 # ip nexthop flush
 Dump was interrupted and may be inconsistent.
 Flushed 65536 nexthops
 # ip nexthop | wc -l
 0

Reported-by: Maksym Yaremchuk <maksymy@nvidia.com>
Tested-by: Maksym Yaremchuk <maksymy@nvidia.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Petr Machata <petrm@nvidia.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>