nexthop: Do not increment dump sentinel at the end of the dump

The nexthop and nexthop bucket dump callbacks previously returned a
positive return code even when the dump was complete, prompting the core
netlink code to invoke the callback again, until returning zero.

Zero was only returned by these callbacks when no information was filled
in the provided skb, which was achieved by incrementing the dump
sentinel at the end of the dump beyond the ID of the last nexthop.

This is no longer necessary as when the dump is complete these callbacks
return zero.

Remove the unnecessary increment.

Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Petr Machata <petrm@nvidia.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Link: https://lore.kernel.org/r/20230813164856.2379822-3-idosch@nvidia.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>